Premium
Nonconventional partitioning of the many‐body Hamiltonian for studying correlation effects
Author(s) -
Surján Péter R.,
Kállay Mihály,
Szabados Ágnes
Publication year - 1998
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/(sici)1097-461x(1998)70:4/5<571::aid-qua3>3.0.co;2-s
Subject(s) - hamiltonian (control theory) , wave function , electronic correlation , chemistry , atomic orbital , excitation , quantum mechanics , biorthogonal system , statistical physics , electron , physics , atomic physics , mathematics , mathematical optimization , wavelet transform , artificial intelligence , computer science , wavelet
For the treatment of electron correlation, one most often uses the 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 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 MP n 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 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 equation‐of‐motion technique leads to Tamm–Dankoff approach (TDA) equations which account for correlation effects in electronic excitation energies. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 571–581, 1998