22 MULTIREFERENCE RAYLEIGH SCHRÖDINGER PERTURBATION THEORY

Bibliography:

Original RS2/RS3:

H.-J. Werner, Mol. Phys. 89, 645-661 (1996)

New internally contracted RS2C:

P. Celani and H.-J. Werner, J. Chem. Phys. 112, 5546 (2000)

All publications resulting from use of this program must acknowledge the above.

The commands

RS2,options
RS2C,options
RS3,options

are used to perform second or third-order perturbation calculations. RS3 always includes RS2 as a first step. For closed-shell single-reference cases, this is equivalent to MP2 or MP3 (but a different program is used). RS2C calls a new more efficient second-order program (see below), which should normally be used if third-order is not required (note that RS3C is not available).

Options can be the following:

Gn

Use modified zeroth order Hamiltonian, see section 22.4

SHIFT=value

Level shift, see section 22.5

MIX=nstates

Invokes multi-state (MS-CASPT2) treatment using nstates states. See section 22.3 for more details.

ROOT=ioptroot

Root number to be optimized in geometry optimization. This refers to the nstates included in the MS-CASPT2. See section 22.7 for more details.

SAVEH=record

Record for saving the effective Hamiltonian in MS-CASPT2 calculations. If this is not given, a default record will be used (recommended).

INIT

(logical) Initializes a MS-CASPT2 with single state reference functions, see section 22.3

IGNORE

(logical) Flags an approximate gradient calculation without CP-CASPT2; see section 22.7 for details.

In addition, all valid options for MRCI can be given (see Sect. 21).