Atom-Based Strong Correlation Method: An Orbital Selection Algorithm.

The present study proposes a methodology that advances the selection of initial orbitals for subsequent use in correlation calculations. The initial orbital sets used herein are split-localized orbitals that span a full-valence orbital space and were developed in a previous study ( J. Chem. Phys. 2013 , 139 , 234107 ) in order to reveal the bonding patterns of molecules in a specific, quantitative manner. On the basis of the quantitative chemical features of these localized orbitals, this new method systematically extracts orbital sets and assigns excitation levels that systematically recover strong correlation with smaller numbers of configurations than can be achieved with traditional as well as nontraditional correlation methods. Moreover, this method not only provides organized initial orbitals for correlation calculations but also results in compact configuration interaction expansions via the use of the split-localized orbitals.