# The MRCI program

Multiconfiguration reference internally contracted configuration interaction

Bibliography:

H.-J. Werner and P.J. Knowles, J. Chem. Phys. 89, 5803 (1988).
P.J. Knowles and H.-J. Werner, Chem. Phys. Lett. 145, 514 (1988).

For excited state calculations:

P. J. Knowles and H.-J. Werner, Theor. Chim. Acta 84, 95 (1992).

For explicitly correlated MRCI (MRCI-F12):

T. Shiozaki, G. Knizia, and H.-J. Werner, J. Chem. Phys. 134, 034113 (2011);
T. Shiozaki and H.-J. Werner, J. Chem. Phys. 134, 184104 (2011);
T. Shiozaki and H.-J. Werner, Mol. Phys. 111, 607 (2013).

All publications resulting from use of the corresponding methods must acknowledge the above.

The first internally correlated MRCI program was described in:

H.-J. Werner and E.A. Reinsch, J. Chem. Phys. 76, 3144 (1982).
H.-J. Werner, Adv. Chem. Phys. 59, 1 (1987).

The command CI or CI-PRO or MRCI calls the MRCI program.
The command MRCI-F12 calls the explicitly correlated MRCI-F12 program.
The command CISD calls fast closed-shell CISD program.
The command QCI calls closed-shell quadratic CI program.
The command CCSD calls closed-shell coupled-cluster program.

The following options may be specified on the command line:

• NOCHECK Do not stop if no convergence.
• DIRECT Do calculation integral direct.
• NOSING Do not include singly external configurations.
• NOPAIR Do not include doubly external configurations (not valid for single reference methods).
• MAXIT=value Maximum number of iterations.
• MAXITI=value Maximum number of microiterations (for internals).
• SHIFTI=value Denominator shift for update of internal configurations.
• SHIFTS=value Denominator shift for update of singles.
• SHIFTP=value Denominator shift for update of doubles.
• THRDEN=value Convergence threshold for the energy.
• THRVAR=value Convergence threshold for the CI-vector. This applies to the square sum of the changes of the CI-coefficients.
• SWAP|NOSWAP If SWAP is given, the MRCI wavefunctions are reordered according to maximum overlap with the reference functions (this only applies in multi-state calculations). The default is NOSWAP, i.e. the states are ordered according to increasing MRCI energy.
• ROTREF=value If value=0 the cluster corrections are not printed for the rotated reference energies (cf. Section cluster corrections for multi-state MRCI). If value=1 all corrections are printed. If value=2 (default) the reference are transformed to maximize the overlap with the final MRCI wavefunctions, and the cluster corrections are computed using these rotated reference states. If value=-1 the 2009.1 behaviour is recovered.
• CIREC=record If given, the program attempts reading the reference vectors from a previous CASSCF calculation. The vectors must have been saved in the CASSCF using SAVE,CIREC=record This will only work if the active spaces in the CASSCF and MRCI are the same.

The internally contracted MRCI program is called by the CI command. This includes as special cases single reference CI, CEPA, ACPF, MR-ACPF and MR-AQCC. For closed-shell reference functions, a special faster code exists, which can be called using the CISD, QCI, or CCSD commands. This also allows to calculate Brueckner orbitals for all three cases (QCI and CCSD are identical in this case).

The explicitly correlated variant is called by the command MRCI-F12, see section explicitly correlated MRCI-F12.

With no further input cards, the wavefunction definition (core, closed, and active orbital spaces, symmetry) corresponds to the one used in the most recently done SCF or MCSCF calculation. By default, a CASSCF reference space is generated. Other choices can be made using the OCC, CORE, CLOSED, WF, SELECT, CON, and RESTRICT cards. The orbitals are taken from the corresponding SCF or MCSCF calculation unless an ORBITAL directive is given. The wavefunction may be saved using the SAVE directive, and restarted using START. The EXPEC directive allows to compute expectation values over one-electron operators, and the TRAN directive can be used to compute transition matrix elements for one-electron properties. Natural orbitals can be printed and saved using the NATORB directive.

For excited state calculations see STATE, REFSTATE, and PROJECT.

Note: All occupations must be given before WF, PAIRSS, DOMAIN, REGION or other directives that need the occupations.

OCC,$n_1,n_2,\ldots,n_8$;

$n_i$ specifies numbers of occupied orbitals (including CORE and CLOSED) in irreducible representation number $i$. If not given, the information defaults to that from the most recent SCF, MCSCF or CI calculation.

CORE,$n_1,n_2,\ldots,n_8$;

$n_i$ is the number of frozen-core orbitals in irrep number $i$. These orbitals are doubly occupied in all configurations, i.e., not correlated. If no CORE card is given, the program uses the same core orbitals as the last CI calculation; if there was none, then the atomic inner shells are taken as core. To avoid this behaviour and correlate all electrons, specify

CORE

CLOSED,$n_1,n_2,\ldots,n_8$

$n_i$ is the number of closed-shell orbitals in irrep number $i$, inclusive of any core orbitals. These orbitals do not form part of the active space, i.e., they are doubly occupied in all reference CSFs; however, in contrast to the core orbitals (see CORE), these orbitals are correlated through single and double excitations. If not given, the information defaults to that from the most recent SCF, MCSCF or CI calculation. For calculations with closed-shell reference function (closed=occ), see CISD, QCI, and CCSD.

ORBIT,name.file,[specifications];

name.file specifies the record from which orbitals are read. Optionally, various specifications can be given to select specific orbitals if name.file contains more than one orbital set. For details see section selecting orbitals and density matrices (ORBITAL, DENSITY). Note that the IGNORE_ERROR option can be used to force MPn or triples calculations with non-canonical orbitals.

The default is the set of orbitals from the last SCF, MCSCF or CI calculation.

The number of electrons and the total symmetry of the wavefunction are specified on the WF card:

WF,elec,sym,spin

where

• elec: is the number of electrons
• sym: is the number of the irreducible representation
• spin: defines the spin symmetry, spin$=2S$ (singlet=0, doublet=1, triplet=2, etc.)

The WF card must be placed after any cards defining the orbital spaces OCC, CORE, CLOSED.

The REF card can be used to define further reference symmetries used for generating the configuration space, see REF.

REF,sym;

This card, which must come after the WF directive, defines an additional reference symmetry used for generating the uncontracted internal and singly external configuration spaces. This is sometimes useful in order to obtain the same configuration spaces when different point group symmetries are used. For instance, if a calculation is done in $C_s$ symmetry, it may happen that the two components of a $\Pi$ state, one of which appears in $A'$ and the other in $A''$, come out not exactly degenerate. This problem can be avoided as in the following example:

for a doublet $A'$ state:

WF,15,1,1;      !define wavefunction symmetry (1)
REF,2;          !define additional reference symmetry (2)

and for the doublet A” state:

WF,15,2,1;      !define wavefunction symmetry (2)
REF,1;          !define additional reference symmetry (1)

For linear geometries the same results can be obtained more cheaply using $C_{2v}$ symmetry,

WF,15,2,1;      !define wavefunction symmetry (2)
REF,1;          !define additional reference symmetry (1)
REF,3;          !define additional reference symmetry (3)

or

WF,15,3,1;      !define wavefunction symmetry (2)
REF,1;          !define additional reference symmetry (1)
REF,2;          !define additional reference symmetry (2)

Each REF card may be followed by RESTRICT, SELECT, and CON cards, in the given order.

SELECT,ref1,ref2,refthr,refstat,mxshrf;

This card is used to specify a reference configuration set other than a CAS, which is the default. Configurations can be defined using CON cards, which must appear after the SELECT card. Alternatively, if ref1 is an existing Molpro record name, the configurations are read in from that record and may be selected according to a given threshold.

The select card must be placed directly after the WF or REF card(s), or, if present, the RESTRICT cards. The general order of these cards is

WF (or REF)
RESTRICT (optional)
SELECT  (optional)
CON  (optional)

• ref1=rec1.file: (rec1$>$2000) The configurations are read in from the specified record. See section saving the CI vectors and information for a gradient calculation about how to save the configurations in the MCSCF calculation. If ref1 is not specified, the program assumes that the configurations are read from subsequent CON cards (see CON).
• ref2=rec2.file: (rec2$>$2000) additional configurations are read from the specified record. If rec2 is negative, all records between rec1 and abs(rec2) are read. All configurations found in this way are merged.
• refthr: Selection threshold for configurations read from disc (records rec1–rec2). This applies to the norm of all CSFs for each orbital configuration.
• refstat: Specifies from which state vector the configurations are selected. This only applies to the case that the configurations were saved in a state-averaged calculation. If refstat is zero or not specified, the configurations are selected from all states. If refstat is greater than zero, then the specified reference state is used. If refstat is less than zero, then all appropriate reference states are used. Lastly, if refstat is of the form istat1.istat2, states istat1 through istat2 are used.
• mxshrf: maximum number of open shells in the selected or generated configurations.

RESTRICT,nmin,nmax,orb$_1$,orb$_2$,$\ldots$orb$_n$;

This card can be used to restrict the occupation patterns in the reference configurations. Only configurations containing between nmin and nmax electrons in the specified orbitals orb$_1$, orb$_2$, $\ldots$, orb$_n$ are included in the reference function. If nmin and nmax are negative, configurations with exactly abs(nmin) and abs(nmax) electrons in the specified orbitals are deleted. This can be used, for instance, to omit singly excited configurations. The orbitals are specified in the form number.sym, where number is the number of the orbital in irrep sym. Several RESTRICT cards may follow each other.

The RESTRICT cards must follow the WF or REF cards to which they apply. The general order of these cards is

WF (or REF)
RESTRICT (optional)
SELECT  (optional)
CON  (optional)

If a RESTRICT cards precedes the WF card, it applies to all reference symmetries. Note that RESTRICT also affects the spaces generated by SELECT and/or CON cards.

CON,$n_1,n_2,n_3,n_4,\ldots$

Specifies an orbital configuration to be included in the reference function. $n_1$, $n_2$ etc. are the occupation numbers of the active orbitals (0,1,or 2). Any number of CON cards may follow each other, but they must all appear directly after a SELECT card.

STATE,nstate,nroot(1),nroot(2),…,nroot(nstate);

nstate is the number of states treated simultaneously; nroot(i) are the root numbers to be calculated. These apply to the order of the states in the initial internal CI. If not specified, nroot(i)=$i$. Note that it is possible to leave out states, i.e.,

STATE,1,2;    ! calculates second state
STATE,2,1,3;  ! calculates first and third state

All states specified must be reasonably described by the internal configuration space. It is possible to have different convergence thresholds for each state (see ACCU card). It is also possible not to converge some lower roots which are included in the list nroot(i) (see REFSTATE card). For examples, see REFSTATE card.

REFSTATE,nstatr,nrootr(1),nrootr(2),…,nrootr(nstatr);

nstatr is the number of reference states for generating contracted pairs. This may be larger or smaller than nstate. If this card is not present, nstatr=nstate and nrootr(i)=nroot(i). Roots for which no reference states are specified but which are specified on the STATE card (or included by default if the nroot(i) are not specified explicitly on the STATE card) will not be converged, since the result will be bad anyway. However, it is often useful to include these states in the list nroot(i), since it helps to avoid root flipping problems. Examples:

state,2;

will calculate two states with two reference states.

state,2;refstate,1,2;

will optimize second state with one reference state. One external expansion vector will be generated for the ground state in order to avoid root flipping. The results printed for state 1 are bad and should not be used (unless the pair space is complete, which might happen in very small calculations).

state,1,2;refstate,1,2;

As the second example, but no external expansion vectors will be generated for the ground state. This should give exactly the same energy for state 2 as before if there is no root flipping (which, however, frequently occurs).

state,2;accu,1,1,1;

Will calculate second state with two reference states. The ground state will not be converged (only one iteration is done for state 1) This should give exactly the same energy for state 2 as the first example.

PAIR,iorb1.isy1,iorb2.isy2,np;

is a request to correlate a given orbital pair.

• np=1: singlet pair
• np=-1: triplet pair
• np=0: singlet and triplet pair (if possible)

Default is to correlate all electron pairs in active and closed orbitals. See also PAIRS card.

PAIRS,iorb1.isy,iorb2.isy,np;

Correlate all pairs which can be formed from orbitals iorb1.isy1 through iorb2.isy2. Core orbitals are excluded. Either iorb2 must be larger than iorb1 or isy2 larger than isy1. If iorb1.isy1=iorb2.isy2 the PAIRS card has the same effect as a PAIR card. PAIR and PAIRS cards may be combined.

If no PAIR and no PAIRS card is specified, all valence orbitals are correlated. The created pair list restricts not only the doubly external configurations, but also the all internal and semi internals.

NOPAIR;

No doubly external configurations are included.

NOSINGLE;

No singly external configurations are included.

NOEXC;

Perform CI with the reference configurations only.

CEPA(ncepa);

($0 \le ncepa \le 3$). Instead of diagonalizing the hamiltonian, perform CEPA calculation, CEPA type ncepa. This is currently available only for single configuration reference functions.

ACPF,options
AQCC,options

where options can be

• GACPFI=gacpfi
• GACPFE=gacpfe

Instead of diagonalizing the hamiltonian, perform ACPF calculation or AQCC calculation. Using the options GACPFI and GAPCPE The internal and external normalization factors gacpfi, gacpfe may be reset from their default values of 1, 2/nelec and 1, 1-(nelec-2)(nelec-3)/nelec(nelec-1), respectively.

The ACPF and related methods are currently not robustly working for excited states. Even though it sometimes works, we do not currently recommend and support these methods for excited state calculations.

PROJECT,record,nprojc;

Initiate or continue a projected excited state calculation, with information stored on record. If nprojc$>0$, the internal CI vectors of nprojc previous calculations are used to make a projection operator. If nprojc$=-1$, this calculation is forced to be the first, i.e. ground state, with no projection. If nprojc$=0$, then if record does not exist, the effect is the same as nprojc$=-1$; otherwise nprojc is recovered from the dump in record. Thus for the start up calculation, it is best to use project,record,-1; for the following excited calculations, use project,record; At the end of the calculation, the wavefunction is saved, and the information in the dump record updated. The project card also sets the tranh option, so by default, transition hamiltonian matrices are calculated.

For example, to do successive calculations for three states, use

ci;...;project,3000.3,-1;
ci;...;project,3000.3;
ci;...;project,3000.3;

At the end of each calculation, Hamiltonian is diagonalized over the whole set of projected functions, and the diagonal and transition properties are transformed accordingly. The untransformed properties, if required, can be retrieved from the output.

TRANH,option;

If option$>-1$, this forces calculation of transition hamiltonian matrix elements in a TRANS or PROJECT calculation. If option$<1$, this forces calculation of one electron transition properties.

ACCU,istate,energy,coeff;

Convergence thresholds for state istate. The actual thresholds for the energy and the CI coefficients are 10**(-energy) and 10**-(coeff). If this card is not present, the thresholds for all states are the default values or those specified on the THRESH card.

SHIFT,shiftp,shifts,shifti;

Denominator shifts for pairs, singles, and internals, respectively.

MAXITER,maxit,maxiti;

• maxit: maximum number of macroiterations;
• maxiti: maximum number of microiterations (internal CI).

MAXDAV,maxdav,maxvi;

• maxdav: maximum number of external expansion vectors in macroiterations;
• maxvi: maximum number of internal expansion vectors in internal CI.

PSPACE,select,npspac;

• select: energy criterion for selecting p-space configurations. If negative, a test for p-space H is performed.
• npspac: minimum number of p-space configurations. Further configurations are added if either required by select or if configurations are found which are degenerate to the last p-space configuration. A minimum number of npspace is automatically determined from the state specifications.

FOCK,$n_1,n_2,\ldots$;

External orbitals are obtained as eigenfunctions of a Fock operator with the specified occupation numbers $n_i$. Occupation numbers must be provided for all valence orbitals.

SAVE,savecp,saveco,idelcg;

or

SAVE [,CIVEC=savecp] [,CONFIG=saveco] [,DENSITY=dumprec] [,NATORB=dumprec] [,FILES] [,SPINDEN]

• savecp: record name for save of wavefunction. If negative the wavefunction is saved after each iteration, else at the end of the job. In case of coupled cluster methods (CCSD, QCISD, BCCD), the wavefunction is saved in each iteration in any case.
• saveco: record name for save of internal configurations and their maximum weight over all states for subsequent use as reference input (see SELECT card). If the record already exists, the record name is incremented by one until a new record is created.
• idelcg: if nonzero or FILES is specified, don’t erase icfil and igfil (holding CI and residual vectors) at the end of the calculation.
• dumprec: Dump record for saving density matrix and natural orbitals. Only one dump record must be given. In any case the density matrix and the natural orbitals are saved. See also DM or NATORB cards.
• SPINDEN In the second form, if specified, spin-density is evaluated and stored. It is not done by default, due to additional required memory and CPU time.

START,readc1,irest;

• readc1: record name from which the wavefunction is restored for a restart. In the case of coupled cluster methods (CCSD, QCISD, BCCD), the amplitudes are read from record readc1 and used for restart.
• irest: If nonzero, the CI coefficients are read and used for the restart; otherwise, only the wavefunction definition is read in.

EXPEC,oper$_1$,oper$_2$,oper$_3$,…;

After the wavefunction determination, calculate expectation values for one-electron operators oper$_i$. See section One-electron operators and expectation values (GEXPEC) for the available operators and their keywords. In multi-state calculations or in projected calculations, also the transition matrix elements are calculated.

TRANS,readc1,readc2,[BIORTH],[oper$_1$,oper$_2$,oper$_3$,…];

Instead of performing an energy calculation, only calculate transition matrix elements between wavefunctions saved on records readc1 and readc2. See section One-electron operators and expectation values (GEXPEC) for a list of available operators and their corresponding keywords. If no operator names are specified, the dipole transition moments are calculated.

If option BIORTH is given, the two wavefunctions may use different orbitals. However, the number of active and inactive orbitals must be the same in each case. Note that BIORTH is not working for spin-orbit matrix elements. Under certain conditions it may happen that biorthogonalization is not possible, and then an error message will be printed. For transition properties which have nonzero nuclear contribution, the corresponding geometry is then that of the ket state (readc2). The same is valid when origin of operator is explicitly specified as a center number, i.e. its coordinates will be those for the ket state.

DM,record.ifil,[idip];

The first order density matrices for all computed states are stored in record record on file ifil. If idip is not zero, the dipole moments are printed starting at iteration idip. See also NATORB. In case of transition moment calculation, the transition densities are also stored, provided both states involved have the same symmetry.

NATORB,[RECORD=]record.ifil,[PRINT=nprint],[CORE[=natcor]];

Calculate natural orbitals. The number of printed external orbitals in any given symmetry is nprint) (default 2). nprint=-1 suppressed the printing. If record is nonzero, the natural orbitals and density matrices for all states are saved in a dump record record on file ifil. If record.ifil is specified on a DM card (see above), this record is used. If different records are specified on the DM and NATORB cards, an error will result. The record can also be given on the SAVE card. If CORE is specified, core orbitals are not printed.

Note: The dump record must not be the same as savecp or saveco on the SAVE card, or the record given on the PROJECT.

OPTION,code1=value,code2=value,$\ldots$

Can be used to specify program parameters and options. If no codes and values are specified, active values are displayed. The equal signs may be omitted. The following codes are allowed (max 7 per card):

• NSTATE: see state card
• NSTATI: number of states calculated in internal CI
• NSTATR: see refstat card
• NCEPA: see CEPA card
• NOKOP: if nonzero, skip integral transformation
• ITRDM: if .ge. 0 transition moments are calculated
• ITRANS: if nonzero, perform full integral transformation (not yet implemented)
• IDIP: Print dipole moments from iteration number value
• REFOPT: if nonzero, optimize reference coefficients; otherwise extract reference coefficients from internal CI
• IAVDEN: average HII and HSS denominators over spin couplings if nonzero
• IDELCG: if.ne.0 then destroy files icfil,igfil at end
• IREST: if nonzero, restart
• NATORB: if nonzero, natural orbitals are calculated and printed. The number of printed external orbitals per symmetry is min(natorb,2)
• WFNAT: if nonzero, natural orbitals are saved to this record
• IPUNRF: if nonzero, punch coefficients of reference configurations
• NPUPD: if nonzero, update pairs in nonorthogonal basis, otherwise in orthogonal basis.
• MAXIT: see maxiter card
• MAXITI: see maxiter card
• MAXDAV: see maxdav card
• MAXVI: see maxdav card
• NOSING: see nosing card
• NOPAIR: see nopair card
• MXSHRF: see select card
• IKCPS=0: In CIKEXT, only K(CP) is calculated; this option taken when and only when no singles.
• IKCPS=1: only K(CP’) is calculated. Implies that modified coupling coefficients are used.
• IKCPS=2: K(CP) and K(CP’) are calculated. Default is IKCPS=2 except when single reference configuration, when IKCPS=1.
• IOPTGM: Option for density matrix routines.
• IOPTGM=0: all quantities in density matrix routines are recalculated for each intermediate symmetry (max. CPU, min. core).
• IOPTGM=1: quantities precalculated and stored on disk (max. I/O, min. core).
• IOPTGM=2: quantities precalculated and kept in core (min. CPU, max. core).
• IOPTOR: If nonzero, calculate intermediate orbitals for each pair. Might improve convergence in some cases, in particular if localized orbitals are used.

PARAM,code1=value,code2=value$\ldots$

Redefine system parameters. If no codes are specified, the default values are displayed. The following codes are allowed:

• LSEG: disc sector length
• INTREL: number of integers per REAL*8 word
• IVECT=0: scalar machine
• IVECT=1: vector machine
• MINVEC: call MXMB in coupling coefficient routines if vector length larger than this value.
• IBANK: number of memory banks for vector machines. If IBANK$>$1, vector strides which are multiples of IBANK are avoided where appropriate.
• LTRACK: number of REAL*8 words per track or block (for file allocation)
• LTR: determines how matrices are stored on disc. If LTR=LSEG, all matrices start at sector boundaries (which optimizes I/O), but unused space is between matrices (both on disc and in core). With LTR=1 all matrices are stored dense. This might increase I/O if much paging is necessary, but reduce I/O if everything fits in core.
• NCPUS: Maximum number of CPUs to be used in multitasking.

THRESH,code1=value,code2=value$\ldots$

If value=0, the corresponding threshold is set to zero, otherwise 10**(-value). The equal signs may be omitted. If no codes are specified, the default values are printed. The following codes are allowed (max 7 per card):

• ZERO: numerical zero
• THRDLP: delete pairs if eigenvalue of overlap matrix is smaller than this threshold.
• PNORM: delete pair if its norm is smaller than this threshold (all pairs are normalized to one for a closed shell case).
• PRINTCI: print CI coefficients which are larger than this value.
• INTEG: omit two-electron integrals which are smaller than this value.
• ENERGY: convergence threshold for energy; see also: ACCU card.
• COEFF: convergence threshold for coefficients; see also: ACCU card.
• SPARSE: omit coefficient changes which are smaller than this value.
• EQUAL: set values in the internal vector and the diagonal elements equal if they differ by less than this value. Useful for keeping track of symmetry.

PRINT,code1=value,code2=value,$\ldots$

Print options. Generally, the value determines how much intermediate information is printed. value=-1 means no print (default for all codes). In some of the cases listed below the specification of higher values will generate even more output than described. The equal signs and zeros may be omitted. All codes may be truncated to three characters. The following codes are allowed (max 7 per card):

• ORBITALS: print orbitals
• JOP=0: print operator list
• JOP=1: print coulomb operators in MO basis
• JOP=2: print coulomb operators in AO and MO basis
• KOP: as JOP for internal exchange operators
• KCP=0: print paging information for CIKEXT
• KCP=1: print external exchange operators in MO basis
• KCP=2: print operators in AO and MO basis
• DM=0: print paging information for CIDIMA
• DM=1: print density matrix in MO basis
• DM=2: print density matrix in AO and MO basis
• FPP=0: print energy denominators for pairs
• FPP=1: in addition, print diagonal coupling coefficients in orthogonal basis.
• FPP=2: print operators FPP
• CP=0: print update information for pairs in each iteration
• CP=1: print pair matrix updates (MO basis)
• CP=2: in addition print pair matrices (MO basis)
• CP=3: print CP in AO basis (in CIKEXT)
• CI=0: print convergence information for internal CI
• CI=1: print internal CI coefficients and external expansion coefficients
• CS: as CP for singles
• CPS=0: print paging information for CICPS
• CPS=1: print matrices CPS in MO basis
• GPP=0: print paging information for CIGPQ
• GPP=1: print matrices GP at exit of CIGPQ
• GPS=0: print paging information for CIGPS
• GPS=1: print vectors GS at exit CIGPS
• GSP=1: print matrices GP at exit CIGPS
• GPI=0: print paging information for CIGPI
• GPI=1: print total GP in orthogonal basis
• GPI=2: print matrices GP and TP
• GIP=0: print paging information for CIGIP
• GIP=1: print GI at exit CIGIP
• GSS=0: print paging information for CIGSS
• GSS=1: print vectors GS at exit CIGSS
• GSI=0: print paging information for CIGSI
• GSI=1: print GS at exit CIGSI
• GIS=0: print paging information for CIGIS
• GIS=1: print GI at exit CIGIS
• GII: print intermediate information in internal CI
• DPQ: print coupling coefficients $\alpha(P,Q)$
• EPQ: print coupling coefficients $\beta(P,Q)$
• HPQ: print coupling coefficients $\gamma(P,Q)$
• DPI: print coupling coefficients for pair-internal interactions
• DSS: not yet used
• DSI: not yet used
• LOG: At end of each iteration, write summary to log file. Delete at end of job if LOG=0
• CC=0: print address lists for coupling coefficients
• CC=1: print coupling coefficients
• DEN=1: print internal first order density
• DEN=2: print internal second order density
• DEN=3: print internal third order density
• DEN=4: print first, second and third order densities
• GAM=1: print first order transition densities
• GAM=2: print second order transition densities
• GAM=3: print first and second order transition densities
• PAIRS=0: print list of non redundant pairs
• PAIRS=1: print list of all pairs
• CORE=0: print summary of internal configurations ($N, N-1$ and $N-2$ electron)
• CORE=1: print internal configurations ($N, N-1, N-2$)
• REF=0: print summary of reference configurations
• REF=1: print reference configurations and their coefficients
• PSPACE: print p-space configurations
• HII: print diagonal elements for internals
• HSS: print diagonal elements for singles
• SPQ: various levels of intermediate information in pair orthogonalization routine.
• TEST=0: print information at each subroutine call
• TEST=1: print in addition information about I/O in LESW, SREIBW
• TEST=2: print also information about I/O in FREAD, FWRITE
• CPU: print analysis of CPU and I/O times
• ALL: print everything at given level (be careful!)
examples/h2o_cepa1.inp
***,Single reference CISD and CEPA-1 for water
r=0.957,angstrom
theta=104.6,degree;
geometry={O;              !z-matrix geometry input
H1,O,r;
H2,O,r,H1,theta}
{hf;wf,10,1;}                         !TOTAL SCF ENERGY     -76.02680642
{ci;occ,3,1,1;core,1;wf,10,1;}        !TOTAL CI(SD) ENERGY  -76.22994348
{cepa(1);occ,3,1,1;core,1;wf,10,1;}   !TOTAL CEPA(1) ENERGY -76.23799334
examples/h2op_mrci_trans.inp
***,Valence multireference CI for X and A states of H2O
gthresh,energy=1.d-8
r=0.957,angstrom,theta=104.6,degree;
geometry={O;              !z-matrix geometry input
H1,O,r;
H2,O,r,H1,theta}
{hf;wf,10,1;}         !TOTAL SCF ENERGY               -76.02680642
{multi;occ,4,1,2;closed,2;frozen,1;wf,9,2,1;wf,9,1,1;tran,ly}
!MCSCF ENERGY                   -75.66755631
!MCSCF ENERGY                   -75.56605896
{ci;occ,4,1,2;closed,2;core,1;wf,9,2,1;save,7300.1}
!TOTAL MRCI ENERGY              -75.79831209
{ci;occ,4,1,2;closed,2;core,1;wf,9,1,1;save,7100.1}
!TOTAL MRCI ENERGY              -75.71309879
{ci;trans,7300.1,7100.1,ly}
!Transition moment <1.3|X|1.1> = -0.14659810  a.u.
!Transition moment <1.3|LY|1.1> = 0.96200488i a.u.
examples/bh_mrci_sigma_delta.inp
***,BH singlet Sigma and Delta states
r=2.1
geometry={b;h,b,r}
{hf;occ,3;wf,6,1;}
{multi;
occ,3,1,1;frozen,1;wf,6,1;state,3;lquant,0,2,0;wf,6,4;lquant,2;
tran,lz;
expec2,lzlz;}
! Sigma states:- energies -25.20509620 -24.94085861
{ci;occ,3,1,1;core,1;wf,6,1;state,2,1,3;}
! Delta states:- energies -24.98625171
{ci;occ,3,1,1;core,1;wf,6,1;state,1,2;}
! Delta state:- xy component
{ci;occ,3,1,1;core,1;wf,6,4;}

In the following, we assume that \begin{align} \Psi_{\rm ref}^{(n)} &=& \sum_R C_{Rn}^{(0)} \Phi_R \\ \Psi_{\rm mrci}^{(n)} &=& \sum_R C_{Rn} \Phi_R + \Psi_c\end{align} are the normalized reference and MRCI wave functions for state $n$, respectively. $C_R^{(0)}$ are the coefficients of the reference configurations in the initial reference functions and $C_{Rn}$ are the relaxed coefficients of these configurations in the final MRCI wave function. $\Psi_c$ is the remainder of the MRCI wave function, which is orthogonal to all reference configurations $\Phi_R$.

The corresponding energies are defined as \begin{align} E_{\rm ref}^{(n)} &=& \langle \Psi_{\rm ref}^{(n)} | \hat H | \Psi_{\rm ref}^{(n)}\rangle,\\ E_{\rm mrci}^{(n)}&=& \langle \Psi_{\rm mrci}^{(n)} | \hat H | \Psi_{\rm mrci}^{(n)}\rangle.\end{align} The standard Davidson corrected correlation energies are defined as \begin{align} E^{n}_{\rm D} &=& E_{\rm corr}^{(n)}\cdot \frac {1-c_n^2}{ c_n^2 } \label{eq:dav}\end{align} where $c_n$ is the coefficient of the (fixed) reference function in the MRCI wave function: \begin{align} c_n = \langle \Psi_{\rm ref}^{(n)}|\Psi_{\rm mrci}^{(n)} \rangle = \sum_R C_{Rn}^{(0)} C_{Rn}, \label{eq:cfix}\end{align} and the correlation energies are \begin{align} E_{\rm corr}^{(n)}=E_{\rm mrci}^{(n)}-E_{\rm ref}^{(n)} .\label{eq:ecorr}\end{align} In the vicinity of avoided crossings this correction may give unreasonable results since the reference function may get a small overlap with the MRCI wave function. One way to avoid this problem is to replace the reference wave function $\Psi_{\rm ref}^{(n)}$ by the the relaxed reference functions \begin{align} \Psi_{\rm rlx}^{(n)} &=& \frac{\sum_R C_{Rn} \Phi_R}{\sqrt{\sum_R C_{Rn}^2}},\end{align} which simply leads to \begin{align} c_n^2 &=& \sum_R C_{Rn}^2. \label{eq:crlx} \end{align} Alternatively, one can linearly combine the fixed reference functions to maximize the overlap with the MRCI wave functions. This yields projected functions \begin{align} \Psi_{\rm prj}^{(n)} &=& \sum_m |\Psi_{\rm ref}^{(m)}\rangle\langle \Psi_{\rm ref}^{(m)} |\Psi_{\rm mrci}^{(n)} \rangle = \sum_m |\Psi_{\rm ref}^{(m)}\rangle d_{mn}\end{align} with \begin{align} d_{mn} &=& \langle \Psi_{\rm ref}^{(m)}|\Psi_{\rm mrci}^{(n)} \rangle = \sum_R C_{Rm}^{(0)} C_{Rn}.\end{align} These projected functions are not orthonormal. The overlap is \begin{align} \langle \Psi_{\rm prj}^{(m)} | \Psi_{\rm prj}^{(n)} \rangle &=& ({\bf d}^{\dagger} {\bf d})_{mn}.\end{align} Symmetrical orthonormalization, which changes the functions as little as possible, yields \begin{align} \Psi_{\rm rot}^{(n)} &=& \sum_m |\Psi_{\rm ref}^{(m)}\rangle u_{mn}, \\ {\bf u} &=& {\bf d} ({\bf d}^{\dagger} {\bf d})^{-1/2}.\end{align} The overlap of these functions with the MRCI wave functions is \begin{align} \langle \Psi_{\rm rot}^{(m)} | \Psi_{\rm mrci}^{(n)} \rangle &=& [({\bf d}^{\dagger} {\bf d}) ({\bf d}^{\dagger} {\bf d})^{-1/2}]_{mn} = [({\bf d}^{\dagger} {\bf d})^{1/2}]_{mn}.\end{align} Thus, in this case we use for the Davidson correction \begin{align} c_n &=& [({\bf d}^{\dagger} {\bf d})^{1/2}]_{nn}. \label{eq:crot} \end{align} The final question is which reference energy to use to compute the correlation energy used in eq. \eqref{eq:dav}. In older MOLPRO version (to 2009.1) the reference wave function which has the largest overlap with the MRCI wave function was used to compute the reference energy for the corresponding state. But this can lead to steps of the Davidson corrected energies if the order of the states swaps along potential energy functions. In this version there are two options: the default is to use for state $n$ the reference energy $n$, cf. eq. \eqref{eq:ecorr} (assuming the states are ordered according to increasing energy). The second option is to recompute the correlation energies using the rotated reference functions \begin{align} E_{corr}^{(n)} &=& E_{\rm MRCI}^{(n)} - \langle \Psi_{\rm rot}^{(n)} | \hat H | \Psi_{\rm rot}^{(n)} \rangle \label{eq:erot}\end{align} Both should give smooth potentials (unless at conical intersections or crossings of states with different symmetries), but there is no guarantee that the Davidson corrected energies of different states don’t cross. This problem is unavoidable for non-variational energies. The relaxed and rotated Davidson corrections give rather similar results; the rotated one yields somewhat larger cluster corrections and was found to give better results in the case of the F + H$_2$ potential [see J. Chem. Phys. 128, 034305 (2008)].

By default, the different cluster corrections listed in the following table are computed in multi-state MRCI calculations. and stored in variables.

Cluster corrections computed in multi-state MRCI calculations
Name $c_n$ (Eq.) $E_{corr}^{(n)}$ (Eq.) Variable
Using standard reference energies:
Fixed \eqref{eq:cfix} \eqref{eq:ecorr} ENERGD1(n)
Relaxed \eqref{eq:crlx} \eqref{eq:ecorr} ENERGD0(n)
Rotated \eqref{eq:crot} \eqref{eq:ecorr} ENERGD2(n)
Using rotated reference energies:
Relaxed \eqref{eq:crlx} \eqref{eq:erot} ENERGD3(n)
Rotated \eqref{eq:crot} \eqref{eq:erot} ENERGD4(n)

By default, the energies are in increasing order of the MRCI total energy. In single-state calculations only the fixed and relaxed values are available.

By default, ENERGD(n)=ENERGD0(n). This can be changed by setting OPTION,CLUSTER=x; then ENERGD(n)=ENERGDx(n) (default $x=0$). The behaviour of Molpro 2009.1 and older can be retrieved using

MRCI,SWAP,ROTREF=-1.

The only change needed for including explicitly correlated terms is to append -F12 to the MRCI command. All other options work as described before. It is recommended to use correlation consistent basis sets (aug-cc-pVnZ or VnZ-F12) since for these the appropriate fitting and RI auxiliary basis sets are chosen automatically. Otherwise it max be necessary to specify these basis sets as described for single-reference methods in section explicitly correlated methods.

The following options (to be given on the MRCI-F12 command line) are specific to MRCI-F12:

• SCALF12=scalf12 If scalf12=1 (default) the explicitly correlated contributions are treated variationally (SFIX ansatz, see J. Chem. Phys. 134, 034113 (2011). If scalf12=0 intermediate normalization is used (FIX ansatz).
• CABS_SINGLES=isingles isingles=1: use singles-CI CABS singles correction (default); isingles=2: use perturbational CABS singles correction; isingles=3: include couplings of CABS singles with MRCI terms.

For a description of the various singles corrections see Mol. Phys. 111, 607 (2013). [sec:rs23]