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Related Experiment Videos

Intruder state avoidance multireference Møller-Plesset perturbation theory.

Henryk A Witek1, Yoong-Kee Choe, James P Finley

  • 1Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Tokyo, Japan 113-8656.

Journal of Computational Chemistry
|July 13, 2002
PubMed
Summary

A new intruder state avoidance (ISA) method improves perturbative calculations by modifying Hamiltonians to prevent singularities. This approach enhances convergence for complex molecular systems, particularly when using multireference Møller-Plesset theory.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Perturbative methods in quantum chemistry can suffer from slow convergence or singularities.
  • These issues often arise from small energy denominators, caused by intruder states that inappropriately mix with the target electronic state.

Purpose of the Study:

  • To introduce a novel perturbation approach, intruder state avoidance (ISA), designed to enhance the convergence of low-order perturbative calculations.
  • To address and mitigate the problem of singularities in multireference Møller-Plesset (MRMP) calculations caused by intruder states.

Main Methods:

  • The proposed ISA method modifies the zeroth-order Hamiltonian to enlarge small energy denominators.
  • This technique is applied in conjunction with multireference Møller-Plesset (MRMP) perturbation theory.

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  • Calculations were performed on various molecular systems, including O(2), AgH, N(2), CO, formamide, CH(2), benzene, and porphyrin derivatives.

    • Main Results:

    • The ISA method successfully resolved singularities in MRMP calculations for systems with intruder states, such as the 1(3)Sigma(-)(u) state of O(2) and several states of AgH.
    • The approach demonstrated effectiveness for other systems influenced by intruder states, including N(2), CO, and formamide.
    • In cases without intruder states, ISA showed minimal to no effect on MRMP results, validating its specific purpose.

    Conclusions:

    • The intruder state avoidance (ISA) method is a robust technique for improving the reliability and convergence of perturbative quantum chemical calculations.
    • ISA effectively overcomes limitations of conventional MRMP theory when intruder states are present.
    • This method offers a valuable tool for accurate electronic structure calculations of challenging molecular systems.