Is It Worth Running the Hartree-Fock Calculations With Localized Molecular Orbitals Within the Framework of Variational Coupled Cluster Singles Theory?

Abstract

Some years ago, we proved that Variational Coupled Cluster Singles (VCCS) theory can be effectively used to solve the independent particle model, which gave rise to a diagonalization-free self-consistent-field approach. The resulting formulation enables a solution with “a priori” localized orbitals. In the current contribution, we have explored this still unexplored possibility. Starting molecular orbitals were either localized using the Pipek-Mezey procedure or via an incomplete Cholesky decomposition of the density matrix. The Hartree-Fock solution was obtained within a VCCS iterative procedure, with the starting localized molecular orbitals used for the creation of the reference and the singly excited determinants. The same localized basis was kept in each iteration. For a series of medium-sized molecules, we have investigated the convergence behavior of the iterative procedure together with the sparsity of the single-excitation amplitude vector and the corresponding density matrix expressed in the localized basis.