A non-linear approach to configuration interaction: The low-rank CI method (LR CI)

Abstract

An alternative to the linear CI expansion is presented, which is based on the spectral resolution of the double excitation CI coefficients arranged as a symmetric matrix. The resulting energy expression is a quartic function of the new variables, where the double excitation operator is given as T̂ 2 = ∑ I M ω I ( d̂ I ) 2, where d̂ d I is a single excitation operator: d̂ I =∑ I a , d̂ I iaÊ ai . Since M is a small number, the number of variables is reduced considerably compared to normal SDCI, with only little loss in accuracy. A program for non-linear CI calculations has been written and is presented with results for H 2O and N 2. The method can easily be extended to include orbital optimization and cluster terms in the wavefunction, still yielding a fully variational approximation to the Schrödinger equation.