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5.1 Closed-shell correlation methods

For closed-shell calculations the following methods/keys are available:

mp2 !second-order Moeller-Plesset perturbation theory lmp2 !second-order local Moeller-Plesset perturbation theory mp3 !third-order Moeller-Plesset perturbation theory mp4 !fourth-order Moeller-Plesset perturbation theory ci !singles and doubles configuration interaction (using MRCI program) cisd !singles and doubles configuration interaction (using CCSD program) cepa(1) !Coupled electron pair approximation, version 1 cepa(2) !Coupled electron pair approximation, version 2 cepa(3) !Coupled electron pair approximation, version 3 acpf !Averaged couplec pair functional ccsd !Coupled cluster with singles and doubles ccsd(t) !Coupled cluster with singles and doubles and perturbative !treatment of triples bccd !Brueckner coupled cluster with doubles bccd(t) !Brueckner coupled cluster with doubles and perturbative !treatment of triples qcisd !Quadratic configuration interaction with singles and doubles qcisd(t)!Quadratic configuration interaction with perturbative !treatment of triples

One or more of these commands should be given after the Hartree-Fock input. Note that some methods include others as a by-product; for example, it is wasteful to ask for mp2 and mp4, since the MP4 calculation returns all of the second, third and fourth order energies. CCSD also returns the MP2 energy.

By default, only the valence electrons are correlated. To modify the space of uncorrelated inner-shell (core) orbitals, the core directive can be used, on which the numbers of core orbitals in each symmetry are specified in the same way as the occupied orbitals on the occ card. In order to correlate all electrons in a molecule, use

core

without any further arguments (all entries zero). This card must follow the directive for the method, e.g.,

MP2 !second-order Moeller-Plesset perturbation theory core !correlate all electrons

Note, however, that special basis sets are needed for correlating inner shells, and it does make not much sense to do such calculations with the standard basis sets described above. The correlation-consistent core-valence basis sets (cc-pCV$x$Z where $x$ is D, T, Q, $5,\dots$) are available for this purpose.

MP2 !second-order Moeller-Plesset perturbation theory core,2 !don't correlate electrons in the orbitals 1.1 and 2.1.

The number of occupied orbitals is automatically remembered from the preceding HF calculation. If necessary for special purposes, it can be specified using occ cards as in HF. The orbitals are taken from the most recent HF calculation in the input. See the MOLPRO reference manual for other choices of orbitals.

Example for a complete CCSD(T) calculation for formaldehyde:

***,formaldehyde print,basis,orbitals !this is optional: print the basis set and the occupied orbitals angstrom geometry= !define the nuclear coordinates C O , C , rco H1 , C , rch , O , hco H2 , C , rch , O , hco , H1 , 180

rco=1.182 Ang rch=1.102 Ang hco=122.1789 Degree

basis=vdz !Select basis set hf !Perform HF calculation ccsd(t) !Perform CCSD(T) calculation

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