A new approach to density matrix functional theory

Starting from a pair-excitation multiconfiguration self-consistent field approach considering pairwise excitations of two electrons of opposite spin from a single occupied molecular orbital to a single virtual molecular orbital, we present a natural orbital functional for the electronic energy containing the natural orbitals and the pair-excitation coefficients as variational parameters to be optimized. The occupation numbers of the natural orbitals can be determined from the pair-excitation coefficients in this implicit functional. Test calculations for the water molecule give occupation numbers of the natural orbitals in reasonable agreement with the results of full configuration interaction calculations.