Effect of dynamic electron correlation on a CASSCF potential energy surface with varying covalent and charge transfer electronic characters

The ground state potential energy surfaces of both the penta-2,4-dieniminium cation (PSB3) and the retinal chromophore in rhodopsin feature a low-lying conical intersection surrounded by regions with variable charge-transfer and diradical electronic characters. We investigate the effect of introducing dynamic electron correlation variationally (MRCISD) and perturbatively (MRPT2) along three CASSCF energy paths which scan these regions in PSB3. The result is a stabilization of regions with charge transfer character leading to a reshaping of the CASSCF potential energy surface