Orbital optimized M�ller Plesset second order perturbation theory (OOMP2) can extend standard MP2 to unrestricted and open shell systems for which the Hartree Fock orbitals suffer from spin contamination. By optimizing orbitals with respect to the MP2 energy without the singles correction, OOMP2 gives approximate Brueckner orbitals, typically improving energies and removing spin contamination. Unfortunately, the divergence of the MP2 energy at HOMO-LUMO energy convergence can cause serious problems during orbital optimization by attracting the search to unphysical, divergent minima. We propose and test regularization approaches to make OOMP2 a more robust method with potential for application to smaller gap molecules such as organometallics without fear of divergence.

Regularizing orbital optimized perturbation theory

248th National Meeting (2014)

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