Excitation into the external orbitals provides a means to recover the molecular energy to within chemical accuracy through the recovery of dynamic electron correlation. Natural external orbitals are typically delocalized across the entire molecule and thus give a few leading terms in a compact CI expression, which is followed by a slowly convergent tail with numerous similar energy increments. On the other hand, the atom-centered, quasi-atomic external orbitals1 decrease the CI expansion size by taking advantage of the short-range character of electron correlation. So, maximizing and ordering the quasi-atomic external orbitals based on the corresponding overlap with the internal orbitals on each atom can reduce the cost of CI calculations. However, the resulting quasi-atomic external orbitals require an inter-atomic orthogonalization. CI-SD and CI-SDT calculations are performed on several molecules to demonstrate the effects of different orthogonalization schemes2 on the recovery of dynamic correlation at the doubles or triples level. 1. Aaron C. West, Michael W. Schmidt, Mark S. Gordon, Klaus Ruedenberg. A comprehensive analysis of molecule-intrinsic quasi-atomic, bonding and correlating orbitals. I. Hartree-Fock wave functions. J. Chem. Phys. 2013, 139, 234107. 2. Aaron C. West. Weighted orthogonalization of atomic orbitals: A stable alternative to the Carlson-Keller method. Comp. Theor. Chem. 2014, 1045, 73-77.

Truncating the configuration interaction (CI) expansion through modified orthogonalization of molecular orbitals

254th National Meeting (2017)

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