  The probability of non-radiative transitions in photochemical dynamics is
determined by the derivative couplings, the couplings between different
electronic states through the nuclear degrees of freedom. Efficient and
accurate evaluation of the derivative couplings is, therefore, of central
importance to realize reliable computer simulations of photochemical reactions.
In this work, the derivative couplings for multistate multireference
second-order perturbation theory (MS-CASPT2) and its 'extended' variant
(XMS-CASPT2) are studied, in which we present an algorithm for their analytical
evaluation. The computational costs for evaluating the derivative couplings are
essentially the same as those for calculating the nuclear energy gradients. The
geometries and energies calculated with XMS-CASPT2 for small molecules at
minimum energy conical intersections (MECIs) are in good agreement with those
computed by multireference configuration interaction. As numerical examples,
MECIs are optimized using XMS-CASPT2 for stilbene and a GFP model chromophore
(the 4-para-hydroxybenzylidene-1,2-dimethyl-imidazolin-5-one anion).
