21 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).

for `RS2` an explicitly correlated version as described in
T. Shiozaki and H.-J. Werner, J. Chem. Phys. **133**, 141103 (2010)
is available. This is called using the command

`RS2-F12`, *options*

*Options* can be the following:

`Gn`- Use modified zeroth order Hamiltonian, see section 21.4
`SHIFT=`*value*- Level shift, see section 21.5
`IPEA=`*value*- IPEA shift proposed by G. Ghigo, B. O. Roos, and
P.A. Malmqvist, Chem. Phys. Lett.
**396**, 142 (2004), see section 21.5. `MIX=`*nstates*- Invokes multi-state (MS-CASPT2) treatment using
*nstates*states. See section 21.3 for more details. `XMS=0`:- MS-CASPT2 method of Finley et al. CPL 288, 299 (1998)
`XMS=1`:- Extended multi-state CASPT2 (XMS-CASPT2) method as described in J. Chem. Phys.
**135**, 081106 (2011). fully invariant treatment of level shifts (recommended). `XMS=2`:- XMS-CASPT2 method; level shift is only applied to the diagonal of H0.
`ROOT=`*ioptroot*- Root number to be optimized in geometry optimization. This refers
to the
*nstates*included in the MS-CASPT2. See section 21.7 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 21.3
`IGNORE`- (logical) Flags an approximate gradient calculation without CP-CASPT2; see section 21.7 for details.