Single-Particle Green Function Approach and Correlated Atomic or Molecular Orbitals

In this paper, we propose a generic and systematic approach for study of the electronic structure for atoms or molecules. In particular, we address the issue of single particle states, or orbitals, which should be one of the most important aspects of a quantum many-body theory. We argue that the single-particle Green function provides a most general scheme for generating these single particle states or orbitals. We call them the correlated atomic or molecular orbitals to make a distinction from those determined from Hartree − Fock equation. We present the calculation of the single particle properties (i.e., the electron affinities (EAs) and ionization potentials (IP s)) for the H2O molecule using the correlated molecular orbitals in the context of quantum chemistry with a second-order self energy. We also calculate the total ground state energy with a single Slater wavefunction determined only from the hole states. Comparisons are made with available experimental data as well as with those from the Hartree − Fock or density functional theory (DFT ) calculations. We conclude that the correlated atomic or molecular orbital approach provides a strictest and most powerful method for studying the single-particle properties of atoms or molecules. It also gives a better total energy than do the Hartree − Fock and DFT even at the single Slater determinant level.