RIGIDCI — Propagate initial selection of microstates throughout a geometry optimization.
RIGIDCI
For CI calculations in which a perturbative selection of microstates is used, it may happen that a geometry optimization fails to converge. The source of the problem may be one or more of the following, the last of which is addressed by RIGIDCI.
A crossing between MOs located in and out of the active space. The PROTO=n
keyword may be used to address this problem.
A conical intersection, i.e., a degeneracy of CI eigenstates, one of them being the
primary CI eigenstate being followed during the geometry optimization. The SEPS=n
keyword may be used to address this problem.
Discontinuous nature of the selection of microstates between geometries, which
induces a minor “noise” on the calculated energy but a larger noise on the calculate
gradient. This “noise” can be reduced by increasing the allowed size of the final
microstate space (CIMAX=n
) so that no perturbative selection of
microstates is necessary, but possibly at the expense of a significant decrease in the
heat of formation of the ground state, since non-specific correlation is already
partially accounted for in the semi-empirical SCF parameterization. The only way to
virtually eliminate the “noise” without any local change in the heat of formation is to
use the RIGIDCI keyword, which has the effect of propagating the selection of
microstates determined at the initial geometry throughout the geometry optimization.
However, since RIGIDCI depends on the initial geometry, it may introduce “hysteresis”
effects between different geometry optimization runs on the same molecule, and therefore
should be used with care.
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