Reaction field contribution to the derivative couplings
Dear Molcas developers,
We have been including reaction fields (RF = Kirkwood model) into the computation of CASSCF nonadiabatic couplings (NACs) and molecular gradients. Recently, we discovered some "pathological cases" where the NACs would blow up to unphysical values tens or hundreds of orders of magnitude higher than normally expected. We determined that the origin of this issue is the "The Electronic Reaction Field Contribution" (using the SHOW keyword in Alaska), which is observed to be exactly zero in well-behaved calculations, but takes unreasonable values in the pathological cases. Unfortunately, these pathological cases are rather unreproducible and seem to be sensitive to exact calculation settings (e.g. order of the multipole expansion used for the Kirkwood model) and even depend on the computational environment (e.g. hardware and especially amount of memory). Hence, I have refrained from attaching an explicit example calculation for now.
Considering that the RF contribution to the molecular gradients are always well-behaved (i.e. non-zero and moderate), my question would be whether the usage of Kirkwood+NACs is safe in principle (especially as I could not find a test for that use case), or if it was never really intended to be used together as RF gradients presumably were initially implemented with only molecular gradients in mind. We did some numerical tests in the beginning and found good agreement, which however would also be consistent with a negligible, but in principle non-vanishing RF contribution.
Our observations (i.e. often zero but sometimes seemingly arbitrary values spanning many magnitudes) seem to fit with an improperly initialised array during the evaluation of the RF contribution to the NACs, which does not get populated with the correctly computed values. Or is the RF contribution to the NACs simply expected to be zero? Unfortunately, I didn't have much success so far to work through the RF gradient implementation myself as I was unable to find literature regarding the underlying theory.
Any pointers would be highly appreciated!
Jakob