α‑CASSCF: An Efficient, Empirical Correction for SA-CASSCF To Closely Approximate MS-CASPT2 Potential Energy Surfaces

Because of its computational
efficiency, the state-averaged complete
active-space self-consistent field (SA-CASSCF) method is commonly
employed in nonadiabatic ab initio molecular dynamics. However, SA-CASSCF
does not effectively recover dynamical correlation. As a result, there
can be qualitative differences between SA-CASSCF potential energy
surfaces (PESs) and more accurate reference surfaces computed using
multistate complete active space second-order perturbation theory
(MS-CASPT2). Here we introduce an empirical correction to SA-CASSCF
that scales the splitting between individual states and the state-averaged
energy. We call this the α-CASSCF method, and we show here that
it significantly improves the accuracy of relative energies and PESs
compared with MS-CASPT2 for the chromophores of green fluorescent
and photoactive yellow proteins. As such, this method may prove to
be quite valuable for nonadiabatic dynamics.