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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
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.
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.
- 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.