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# 9 Multi-state CASPT2

Multi-state caspt2 calculations are only possible with the rs2 program.

There are various possibilities how to choose the zeroth-order Hamiltonian and the configuration basis. Here we describe only the recommended method; for more details see manual.

Three options are relevant:

rs2,xms=value,mix=nstates,root=iroot

where

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.
nstates:
Number of states included in the calculation
iroot:
Root number to be optimized in subseqent gradient calculation (only required if gradient calculations follows)

Example:

```***,mscaspt2 for h2o 3B2 states
basis=vdz
geometry={O
H1,O,R;
H2,O,R,H1,THETA}
R=2.4
Theta=98

hf
{multi;closed,2
wf,10,1,2;state,3         !three 3A1 states included in sa-casscf
wf,10,2,2;state,2         !two 3B1 states included in sa-casscf
wf,10,3,2;state,3         !three 3B2 states included in sa-casscf
canonical,2140.2}         !save pseudo canonical orbitals

{rs2,xms=2,mix=3,root=2;  !include three B2 states,
!select the second state for gradients
wf,10,3,2}                !Triplet B2 state symmetry

optg                      !optimize the geometry
```

molpro@molpro.net 2013-05-24