Regularized orbital-optimized second-order perturbation theory.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Orbital-optimized second-order perturbation theory (OOMP2) improves upon traditional methods by optimizing the zeroth-order wave function.
• OOMP2 is crucial for systems with spin-contaminated mean-field orbitals, like open-shell molecules and organometallic compounds.
• A key limitation of OOMP2 is its susceptibility to divergences during optimization when frontier molecular orbital energies approach degeneracy.

Purpose of the Study:

• To address and resolve the divergence issue in OOMP2 calculations.
• To enhance the robustness and applicability of OOMP2 for challenging molecular systems.
• To improve the accuracy of OOMP2 for a range of chemical properties.

Main Methods:

• Introduction of a simple level shift parameter into the denominator of the MP2 amplitudes.
• Systematic testing of the regularized OOMP2 method with a significant level shift (400 mEh).
• Evaluation of the method's performance across diverse chemical benchmarks.

Main Results:

• The added level shift parameter effectively removes divergent behavior in OOMP2 calculations.
• The regularized OOMP2 method demonstrates improved accuracy for atomization energies.
• Enhanced performance is observed for barrier heights, intermolecular interactions, radical stabilization energies, and metal binding energies.

Conclusions:

• The level-shifted OOMP2 method provides a robust and accurate approach for quantum chemical calculations.
• This regularization technique overcomes a major hurdle, expanding the utility of OOMP2.
• The findings suggest OOMP2 with level shifting is a valuable tool for studying complex chemical systems.