MOLPRO

21 MULTIREFERENCE RAYLEIGH SCHRÖDINGER PERTURBATION THEORY 175
rs2,analytical 2.4259 96.7213 -75.81630628
rs2,numerical 2.4259 96.7213 -75.81630628
21.8 Coupling MRCI and MRPT2: The CIPT2 method
P. Celani, H. Stoll, H.-J. Werner and P. J. Knowles, Mol. Phys. 102, 2369 (2004).
For particularly difficult cases with strong intruder problems, or in which second-order perturbation
theory fails to predict reliable results, a new method that couples MRCI and CASPT2 has
been developed. This variant is invoked using the CIPT2 directive:
CIPT2
In this case all excitations solely from active orbitals are treated by MRCI, while the remaining
excitations involving inactive (closed-shell) orbitals are treated by second-order perturbation
theory. Both methods are coupled by minimizing an appropriate energy functional. Of course,
this method is much more expensive that MRPT2. The cost is comparable to the cost for an
MRCI without correlating the inactive orbitals.
21.9 Further options for CASPT2 and CASPT3
Other options can be set using the OPTION command. These options are mainly used for
testing purposes and should be used with care. It should be noted that the only option that can
be modified in the RS2C program is IFDIA: all others only work with RS2/RS3.
OPTION,code1=value,code2=value,...
Of relevance for the CASPT2/3 program are the following options:
IPROCS=0
IPROCS=1
(Default). Calculation uses uncontracted singles with RS2.
Non-interacting singles are projected out during update. This
is an approximate procedure which should be used with care.
IPROCS=2 The singles are fully internally contracted in RS2. This is
achieved via a projection operator during the coefficient update
and may be inefficient. G
IPROCS=3
IPROCI=0
IPROCI=1
IPROCS=3,IPROCI=1
IFDIA=0
IFDIA=1
IFDIA=2
Only singles with one or two holes in the closed-shells are internally
contracted in RS2 using a projection operator.
(Default). Calculation uses uncontracted internals with RS2.
Internals with two holes in the inactive space are internally contracted
in RS2 using a projection operator.
This combination of options reproduces with RS2 the RS2C
result using projection operators. This requires lot of memory
and disk space and it is feasible only for small molecules.
(Default). All off-diagonal elements of the effective Fock matrix
are included.
The internal-external block of the Fock-matrix is neglected.
This eliminates the single-pair coupling.
All off-diagonal elements of the Fock matrix are neglected.
This corresponds to CASPT2D of Andersson et al. Note: in
this case the result is not invariant to rotations among active
orbitals!