22.7 Options for CASPT2 and CASPT3

Other options can be set using the OPTION command. These options are mainly used for testing purposes and should be used with care. It should be noted that the only option that can be modified in the RS2C program is IFDIA: all others only work with RS2/RS3.

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

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.

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.