Classical Force Fields Tailored for QM Applications: Is It Really a Feasible Strategy?

Classical molecular
dynamics is more and more often coupled to
quantum mechanical based techniques as a statistical tool to sample
configurations of molecular systems embedded in complex environments.
Nonetheless, the classical potentials describing the molecular systems
are seldom parametrized to reproduce electronic processes, such as
electronic excitations, which are instead very sensitive to the underlining
description of the molecular structure. Here, we analyze the challenging
case of the peridinin molecule, a natural apocarotenoid responsible
for the light-harvesting process in the PCP antenna protein of dinoflagellates.
Ground-state structural and vibrational properties, as well as electronic
transitions of the pigment are studied by means of quantum-mechanical
static and dynamic calculations. Thereafter, classical molecular dynamics
simulations are performed with a number of different force-fields,
ranging from a popular, general purpose one to refined potentials
of increasing level of complexity. From the comparison of classical
results with their quantum mechanical counterparts, it appears that,
while very poor results are obtained from standard transferrable force-fields,
specifically tuned potentials are able to correctly characterize most
of the structural and vibrational features of the pigment. Nonetheless,
only an advanced parametrization technique is able to give a semiquantitative
description of the coupling between vibrations and electronic excitations,
thus suggesting that the use of classical MD in combination of QM
calculations for the study of photoinduced processes, albeit possible,
should be considered with care.