`OPTION`,*code1=value,code2=value,*

Of relevance for the CASPT2/3 program are the following options:

`IPROCS=0`- (Default). Calculation uses uncontracted singles with
`RS2`. `IPROCS=1`- Non-interacting singles are projected out during update. This is an approximate procedure which should be used with care.
`IPROCS=2`- The singles are fully internally contracted in
`RS2`. This is achieved via a projection operator during the coefficient update and may be inefficient. G `IPROCS=3`- Only singles with one or two holes in the closed-shells are internally
contracted in
`RS2`using a projection operator. `IPROCI=0`- (Default). Calculation uses uncontracted internals with
`RS2`. `IPROCI=1`- Internals with two holes in the inactive space are internally contracted
in
`RS2`using a projection operator. `IPROCS=3,IPROCI=1`- This combination of options
reproduces with
`RS2`the`RS2C`result using projection operators. This requires lot of memory and disk space and it is feasible only for small molecules. `IFDIA=0`- (Default). All off-diagonal elements of the effective Fock matrix are included.
`IFDIA=1`- The internal-external block of the Fock-matrix is neglected. This eliminates the single-pair coupling.
`IFDIA=2`- All off-diagonal elements of the Fock matrix are neglected. This corresponds to CASPT2D of Andersson et al. Note: in this case the result is not invariant to rotations among active orbitals!
`IHINT=0`- (Default). Only one-electron integrals are used in the zeroth-order Hamiltonian for all interactions.
`IHINT=1`- The all-internal two-electron integrals are used in the zeroth-order Hamiltonian for the internal-internal and single-single interactions.
`IHINT=2`- The all-internal two-electron integrals in the zeroth-order Hamiltonian
are used for the internal-internal, single-single, and pair-pair interactions. Using
`IHINT=2`and`IDFIA=1`corresponds to Dyall's CAS/A method for the case that CASSCF references with no closed-shells (inactive orbitals) are used. Note that this requires more CPU time than a standard CASPT2 calculation. Moreover, convergence of the CAS/A method is often slow (denominator shifts specified on a`SHIFT`card may be helpful in such cases). In general, we do not recommend the use of`IHINT`with nonzero values. `NOREF=1`- (Default). Interactions between reference configurations and singles are omitted.
`NOREF=0`- Interactions between reference configurations and singles are included. This causes a relaxation of the reference coefficients but may lead to intruder-state problems.
`IMP3=2`- After CASPT2 do variational CI using all internal configurations and the first-order wavefunctions of all states as a basis. In this case the second-order energy will correspond to the variational energy, and the third-order energy approximately to a Davidson-corrected energy. This is useful in excited state calculations with near-degeneracy situations.