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.