MOLPRO

21 MULTIREFERENCE RAYLEIGH SCHRÖDINGER PERTURBATION THEORY 165
Both should give smooth potentials (unless at conical intersections or crossings of states with
different symmetries), but there is no guarantee that the Davidson corrected energies of different
states don’t cross. This problem is unavoidable for non-variational energies. The relaxed and
rotated Davidson corrections give rather similar results; the rotated one yields somewhat larger
cluster corrections and was found to give better results in the case of the F + H 2 potential [see J.
Chem. Phys. 128, 03430 (2008)].
By default, the different cluster corrections listed in Table 10 are computed in multi-state MRCI
calculations. and stored in variables. By default, ENERGD(n)=ENERGD0(n). This can be
Table 10: Cluster corrections computed in multi-state MRCI calculations. By default, the energies
are in increasing order of the MRCI total energy. In single-state calculations only the fixed
and relaxed values are available.
Name c n (Eq.) E corr (n) (Eq.) Variable
Using standard reference energies:
Fixed (51) (52) ENERGD1(n)
Relaxed (54) (52) ENERGD0(n)
Rotated (61) (52) ENERGD2(n)
Using rotated reference energies:
Relaxed (54) (62) ENERGD3(n)
Rotated (61) (62) ENERGD4(n)
changed by setting OPTION,CLUSTER=x; then ENERGD(n)=ENERGDx(n) (default x = 0).
The behaviour of Molpro 2009.1 and older can be retrieved using
MRCI,SWAP,ROTREF=-1.
21 MULTIREFERENCE RAYLEIGH SCHRÖDINGER PERTUR-
BATION THEORY
Bibliography:
Original RS2/RS3:
H.-J. Werner, Mol. Phys. 89, 645-661 (1996)
New internally contracted RS2C:
P. Celani and H.-J. Werner, J. Chem. Phys. 112, 5546 (2000)
All publications resulting from use of this program must acknowledge the above.
The commands
RS2,options
RS2C,options
RS3,options
are used to perform second or third-order perturbation calculations. RS3 always includes RS2
as a first step. For closed-shell single-reference cases, this is equivalent to MP2 or MP3 (but a
different program is used). RS2C calls a new more efficient second-order program (see below),
which should normally be used if third-order is not required (note that RS3C is not available).
Options can be the following: