Construction of an effective hamiltonian for open‐shell molecular systems

Abstract

AbstractAn effective Hamiltonian for open‐shell molecular systems is constructed. The unrestricted Hartree–Fock orbitals are applied as a basis set of one‐particle functions. This effective Hamiltonian is determined as a simple product of the original total Hamiltonian and the spin annihilator. The second‐quantization formalism and the Feynman–Goldstone diagrammatic technique are used. The resulting effective hamiltonian is composed of zero‐ to four‐particle terms. A possibility of applying the nondegenerate diagrammatic perturbation theory constructed over this effective Hamiltonian is discussed.