MOLPRO

21 MULTIREFERENCE RAYLEIGH SCHRÖDINGER PERTURBATION THEORY 168
CASPT2). There is one contracted reference state for each CASPT2 calculation that is specific
for the state under consideration. This is the cheapest method, but there are no gradients
available in this case. It is the users responsibility to make sure that no state is computed twice.
(ii) All nstates states are treated together, with nstates contracted reference states. This is more
expensive, but should give a more balanced description since the different reference states
can mix in the CASPT2. It is required that nstates equals the number of states specified on
the STATE directive. For this case, denoted ”MS-MR-CASPT2” (multi-state multi reference
CASPT2), analytical energy gradients are available, see section 21.7
21.3.1 Performing SS-SR-CASPT2 calculations
If one wants to mix together nstates CASPT2 wavefunctions, a nstates single-state, singlereference
CASPT2 calculations must be run.
The first calculation must use
{RS2,MIX=nstates, INIT, options
STATE,1,1;}
and the subsequent ones
{RS2,MIX=nstates, options
STATE,1,istate;}
for istate = 2,...,nstates. Further options can be given, for instance a level shift.
At the end of each calculation, the CASPT2 wavefunction is stored, and at the end of the last
CASPT2 calculation the Bloch Hamiltonian and the corresponding overlap matrix are automatically
assembled and printed. The Hamiltonian is diagonalized after symmetrization following
Brandow IJQC 15, 207 (1979), as well as with simple half-sum (averaging). The MS-CASPT2
energy and mixing coefficients printed in each case.
The variable MSENERGY(i) (with i=1,...nstates) is set to the multi-state energies obtained with
half-sum diagonalization. If a Level Shift is present, MSENERGY(i) contains the multi-state
energies obtained with half-sum diagonalization of the Bloch Hamiltonian whose diagonal elements
(CASPT2 energies) have been corrected with the level shift.
Example: SS-SR-CASPT2 calculation for LiF