  The second-order multireference driven similarity renormalization group
perturbation theory (DSRG-MRPT2) theory provides an efficient means of
correcting the dynamical correlation with the multiconfiguration reference
function. The state-averaged DSRG-MRPT2 (SA-DSRG-MRPT2) method is the simplest
means of treating the excited states with DSRG-MRPT2. In this method, the
Hamiltonian dressed with dynamical correlation is diagonalized in the CASCI
state subspace (SA-DSRG-MRPT2c) or the configuration subspace (SA-DSRG-MRPT2).
This work develops the analytical gradient theory for spin-free
SA-DSRG-MRPT2(c) with the density-fitting (DF) approximation. We check the
accuracy of the analytical gradients against the numerical gradients. We
present applications for optimizing minimum energy conical intersections (MECI)
of ethylene and retinal model chromophores (PSB3 and RPSB6). We investigate the
dependence of the optimized geometries and energies on the flow parameter and
reference relaxations. The smoothness of the SA-DSRG-MRPT2(c) potential energy
surfaces near the reference (CASSCF) MECI is comparable to the XMCQDPT2 one.
These results render the SA-DSRG-MRPT2(c) theory a promising approach for
studies of conical intersections.

Chemical Physics