Nonconventional partitioning of the many-body Hamiltonian for studying correlation effects

✍ Scribed by Péter R. Surján; Mihály Kállay; Ágnes Szabados

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 359 KB
• Volume: 70
• Category: Article
• ISSN: 0020-7608
• DOI: 10.1002/(sici)1097-461x(1998)70:4/5<571::aid-qua3>3.0.co;2-s

✦ Synopsis

For the treatment of electron correlation, one most often uses the Ž . Møller᎐Plesset MP partition which defines the zero-order Hamiltonian through the spectral resolution of the Fockian. We investigate how the MP partitioning can be Ž . improved while still using the Hartree᎐Fock HF reference state; and how the HF wave function can be substituted by a correlated one preserving the formal simplicity of the HF-based approach. To improve the MPn result, we introduce a fine tuning of energy denominators replacing the HF orbital energies with the ionization potentials obtained from the second-order Dyson equation. As this equation usually tends to close the gaps, a slight decrease of the denominators is expected, inducing an improvement of low-order correlation energies. We keep the simplicity of the MP partitioning and handle Dyson corrections as simple level shifts. Substituting doubly filled HF orbitals by strongly orthogonal geminals, one introduces a correlated reference state which is variational, size-consistent, and properly describes single-bond dissociation. This wave function, the Ž . antisymmetrized product of strongly orthogonal geminals APSG , offers a good starting point for further corrections. We show that the use of an APSG reference state in the Ž . equation-of-motion technique leads to Tamm᎐Dankoff approach TDA equations which account for correlation effects in electronic excitation energies.