Development of correlation operator approach to model electron correlation

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Vitaly Rassolov of the University of South Carolina is supported by the Chemical Theory, Models and Computational Methods program in the Chemistry Division to develop a novel approach for modeling electron correlation effects as averages over two-particle universal linear operators. This approach, complementary to the increasingly popular Density Functional Theory (DFT) and comparable to it in computational complexity and rigor, may have advantages over DFT: (i) the operators describe excited states with the same level of accuracy as ground states; (ii) it is a natural formalism to model intermolecular forces; (iii) it is compatible with multireference methods, necessary to describe such phenomena as chemical bond-breaking, magnetic interactions in transition metals, and excited states. Rigorous description of correlation interaction via linear operators involves complicated mathematics, thus limiting the approach to small systems. The PI overcomes this by expanding the operators in Gaussian functions, which are a staple in the electronic structure field. The expansions are performed using exact analytic expressions, leaving electronic structure codes with an easy task of working with Gaussians.This research improves and expands computational tools for better understanding and control of chemical reactions, understanding and prediction of intermolecular forces, and description of excited electronic states which are essential for solar energy conversion. Implementation of novel ideas from this research in established software packages will widen their availability to the large community of scientists, in many fields, who utilize quantum chemistry software.