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Efficient Implementation of Approximate Fourth Order N-Electron Valence State Perturbation Theory
Emily M Kempfer1, Kantharuban Sivalingam1, Frank Neese1
1Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr D-45470, Germany.
Journal of Chemical Theory and Computation
|April 4, 2025
Summary
A new fourth-order N-electron-valence perturbation theory (NEVPT4(SD)) method offers improved accuracy for transition metal systems and bond breaking. This computationally affordable method provides significant gains over NEVPT2 for dynamic correlation effects.
Area of Science:
- Quantum chemistry
- Computational chemistry
- Theoretical chemistry
Background:
- N-electron-valence perturbation theory (NEVPT) is a method for calculating electronic structure.
- Higher-order perturbation corrections can improve accuracy but increase computational cost.
- Accurate treatment of dynamic correlation is crucial for systems with significant electron correlation, such as transition metals.
Purpose of the Study:
- To implement and evaluate a partial fourth-order N-electron-valence perturbation theory (NEVPT4(SD)).
- To assess the accuracy and computational cost of NEVPT4(SD) compared to existing methods.
- To investigate the role of higher-order dynamic correlation effects in chemical systems.
Main Methods:
- Implementation of a partial fourth-order NEVPT method (NEVPT4(SD)).
- Inclusion of internally contracted functions spanning the first-order-interacting space (FOIS).
- Evaluation of contributions to second-order in the wave function and fourth-order in the energy, while excluding triple and quadruple excitations from the second-order-interacting space (SOIS).
Main Results:
- NEVPT4(SD) is demonstrated to be perfectly size consistent.
- The method is computationally affordable, comparable to FIC MRCI/MRCEPA(0) and cheaper than FIC MRCC.
- NEVPT4(SD) shows significant accuracy improvements over NEVPT2 for transition metal multiplets and diatomic bond breaking.
- Fourth-order treatment reduces the need for a second d-shell, highlighting its role in capturing higher-order dynamic correlation.
- NEVPT4(SD) offers limited improvement over NEVPT2 for Heisenberg exchange couplings.
Conclusions:
- The NEVPT4(SD) method provides a computationally feasible approach for improving the accuracy of electronic structure calculations.
- It is particularly beneficial for systems with strong electron correlation, such as transition metals.
- While effective for dynamic correlation, its advantage for specific properties like Heisenberg exchange couplings may be limited.
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