Gradients for Configuration Interaction Energies with Spin-Orbit Coupling in a Semiempirical Framework

We present a method for the analytical evaluation of the molecular energy gradient for a semiempirical configuration interaction (CI) wavefunction, taking into account the spin-orbit coupling. We show how to proceed in the simplest case where all the wavefunctions belonging to the CI subspace considered are mixed by the spin-orbit interaction, and we develop an original procedure for the more complex case where only a limited number of CI eigenvectors of the spin-free Hamiltonian are mixed. (C) 2011 Wiley Periodicals, Inc. J Comput Chem 32: 2690-2696,2011