Regularized orbital-optimized second-order perturbation theory.

Orbital-optimized second-order perturbation theory (OOMP2) optimizes the zeroth order wave function in the presence of correlations, removing the dependence of the method on Hartree-Fock orbitals. This is particularly important for systems where mean field orbitals spin contaminate to artificially lower the zeroth order energy such as open shell molecules, highly conjugated systems, and organometallic compounds. Unfortunately, the promise of OOMP2 is hampered by the possibility of solutions being drawn into divergences, which can occur during the optimization procedure if HOMO and LUMO energies approach degeneracy. In this work, we regularize these divergences through the simple addition of a level shift parameter to the denominator of the MP2 amplitudes. We find that a large level shift parameter of 400 mEh removes divergent behavior while also improving the overall accuracy of the method for atomization energies, barrier heights, intermolecular interactions, radical stabilization energies, and metal binding energies.