Family of modified-contracted Schrödinger equations

Abstract

A family of equations that combines contracted Schrödinger equations of different orders is reported here. Attention is focussed on the resulting second order, third order, and fourth order of these modified-contracted Schrödinger equations. Some of these equations are self-contained and have as fixed points those corresponding to the full-configuration interaction eigenstates. The indeterminacy, which hindered initially the use of the contracted Schrödinger equations, does not formally exist in these equations. Relations linking the lower-order reduced density matrices with the higher-order matrices are exactly incorporated into the modified-contracted Schrödinger-equations structure. The cancellation of high-order correlation terms, which is hidden in the contracted Schrödinger equations, now takes an explicit form.