MOLPRO

21 MULTIREFERENCE RAYLEIGH SCHRÖDINGER PERTURBATION THEORY 167
Andersson [Chem. Phys. Lett. 245, 215 (1995)] may also be helpful. Presently, only ”real”
level shifts have been implemented.
With no further input cards, the wavefunction definition (core, closed, and active orbital spaces,
symmetry) corresponds to the one used in the most recently done SCF or MCSCF calculation.
By default, a CASSCF reference space is generated. Other choices can be made using the OCC,
CORE, CLOSED, WF, SELECT, CON, and RESTRICT cards, as described for the CI program.
The orbitals are taken from the corresponding SCF or MCSCF calculation unless an ORBITAL
directive is given.
For a CASPT2 calculation, the zeroth-order Hamiltonian can be brought to a block-diagonal
form when (pseudo)canonical orbitals are used. This leads to fastest convergence. It is therefore
recommended that in the preceding MULTI calculation the orbitals are saved using the
CANONICAL directive (note that the default is NATORB).
Most options for MRCI calculations (like STATE, REFSTATE etc.) apply also for RS2(C) and
RS3 and are not described here again. Some additional options which specific for CASPT2/3
and are described below.
21.2 Excited state calculations
There are two possibilities to perform excited state calculations:
1) One can calculate each state separately. This is done using the card
STATE,1,root
where root is the desired root (i.e., 2 for the first excited state). In this case the Fock operator
used in the zeroth-order Hamiltonian is computed using the density for the given state.
2) Alternatively, two or more states can be computed simultaneously, using
STATE, n [,root1, root2, . . . , rootn]
where n is the number of states to be computed. The default is to compute the lowest n roots.
Optionally, this default can be modified by specifying the desired roots rooti as shown. One
should note that this does not correspond to the multi-state CASPT2 as described in section
21.3.
In the case that several states are computed simultaneously, the fock operator employed in the
zeroth-order Hamiltonian is computed from a state-averaged density matrix, and the zerothorder
Hamiltonians for all states are constructed from the same fock operator. By default, equal
weights for all states are used. This default can be modified using the WEIGHT directive
WEIGHT,w1, w2,. . . ,wn.
If a REFSTATE card is given (see section 20.2.11), the state-averaged fock operator is made for
all reference states, and the WEIGHT card refers to the corresponding states.
21.3 Multi-State CASPT2
Multi-state CASPT2 is implemented as described by Finley et al. CPL 288, 299 (1998). Currently
this can only be used with the RS2 program (i.e., not with RS2C). There are two different
modes in which MS-CASPT2 calculations can be performed:
(i) Each of the states to be mixed is computed independently, and finally all states are mixed. In
the following, such calculations will be denoted SS-SR-CASPT2 (single-state, single reference