On a choice of multireference space in many-body perturbation theory

A method of generating configuration functions without first solving the configuration interaction problem is proposed to form a compact multireference space. A new space of molecular orbitals is built on the basis of the Hartree-Fock equations with orthogonality constraints. The zeroth-order Hamiltonian H0 has been modified so that this new reference function and excited configurations based on it are eigenfunctions of H0. The different versions of H0 for both closed-shell and high-spin open-shell systems are discussed. The method keeps the computational advantages of the genuine Mّller-Plesset perturbation scheme. The calculated results of electron correlation energy and the energy differences with the CI results of some small diatomic molecules are presented to test the performance of the method.