MOLPRO

21 MULTIREFERENCE RAYLEIGH SCHRÖDINGER PERTURBATION THEORY 171
-107
MR-MR-CASPT2 for LiF
-107.05
-107.1
E1_CASPT2
E2_CASPT2
E1_MSCASPT2
E2_MSCASPT2
-107.15
-107.2
3 4 5 6 7 8 9 10
R
One can clearly see that this gives smoother potentials than the SS-SR-CASPT2 calculation in
the previous section. Also, the avoided crossing is shifted to longer distances, which is due to
the improvement of the electron affinity of F.
21.4 Modified Fock-operators in the zeroth-order Hamiltonian.
The g 1 , g 2 , and g 3 operators proposed by Andersson [Theor. Chim. Acta 91, 31 (1995)] as well
as a further g 4 operator may be used. g 4 makes CASPT2 calculations size extensive for cases in
which a molecule dissociates to high-spin open-shell (RHF) atoms.
The index n of the operator to be used is specified on the RS2, RS2C, or RS3 card:
RS2,option
RS2C,option
RS3,option
where option can be G1, G2, G3, or G4. This option can be followed or preceded by other valid
options.
21.5 Level shifts
Level shifts are often useful to avoid intruder state problems in excited state calculations. MOL-
PRO allows the use of shifts as described by Roos and Andersson, [Chem. Phys. Lett. 245, 215
(1995)]. The shift can be specified on the RS2 or RS2C card
RS2 [,Gn] [,SHIFT=shift],IPEA=value
RS2C [,Gn] [,SHIFT=shift],IPEA=value
Typical choices for the shift is are 0.1 − 0.3. Only two figures after the decimal point are
considered. The shift affects the results, the printed energies as well as the ENERGY variable
include the energy correction for the shift as proposed by Roos and Andersson. At convergence,
also the uncorrected energies are printed for comparison.