Approximating the second-order reduced density matrix in terms of the first-order one

Abstract

The information which is contained in an averaged form in the first-order reduced density matrix is sufficient to build a close approximation to the second-order reduced density matrix and consequently to the system energy. This is achieved by using the anticommutating relation of fermion operators and the commutating relation of the pair-of-fermions operators. Three test calculations are reported and analyzed.