  We investigate the molecular geometries of the ground state and the minimal
energy conical intersections (MECIs) between the ground and first excited
states of the models for the retinal protonated Schiff base in the gas phase
using the extended multistate complete active space second-order perturbation
theory (XMS-CASPT2). The biggest model in this work is the rhodopsin
chromophore truncated between the {\epsilon} and {\delta} carbon atoms, which
consists of 54 atoms and 12-orbital {\pi} conjugation. The results are compared
with those obtained by the state-averaged complete active space self-consistent
field (SA-CASSCF). The XMS-CASPT2 results suggest that the minimum energy
conical intersection associated with the so-called 13-14 isomerization is
thermally inaccessible, which is in contrast to the SA-CASSCF results. The
differences between the geometries of the conical intersections computed by
SA-CASSCF and XMS-CASPT2 are ascribed to the fact that the charge transfer
states are more stabilized by dynamical electron correlation than the
diradicaloid states. The impact of the various choices of active spaces, basis
sets, and state averaging schemes is also examined.
