### Abstract

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 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 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.

Original language | English |
---|---|

Pages (from-to) | 571-581 |

Number of pages | 11 |

Journal | International Journal of Quantum Chemistry |

Volume | 70 |

Issue number | 4-5 |

Publication status | Published - 1998 |

### Fingerprint

### Keywords

- Dyson equation
- Excited states
- Geminals
- Level shift
- Partitioning
- Quasi-degenerate PT

### ASJC Scopus subject areas

- Physical and Theoretical Chemistry

### Cite this

**Nonconventional partitioning of the many-body hamiltonian for studying correlation effects.** / Surján, P.; Kállay, M.; Szabados, A.

Research output: Contribution to journal › Article

*International Journal of Quantum Chemistry*, vol. 70, no. 4-5, pp. 571-581.

}

TY - JOUR

T1 - Nonconventional partitioning of the many-body hamiltonian for studying correlation effects

AU - Surján, P.

AU - Kállay, M.

AU - Szabados, A.

PY - 1998

Y1 - 1998

N2 - 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 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 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.

AB - 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 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 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.

KW - Dyson equation

KW - Excited states

KW - Geminals

KW - Level shift

KW - Partitioning

KW - Quasi-degenerate PT

UR - http://www.scopus.com/inward/record.url?scp=0001610878&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0001610878&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:0001610878

VL - 70

SP - 571

EP - 581

JO - International Journal of Quantum Chemistry

JF - International Journal of Quantum Chemistry

SN - 0020-7608

IS - 4-5

ER -