Equation-of-motion regularized orbital-optimized second-order perturbation theory with the density-fitting...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate computation of excitation energies is crucial for understanding molecular properties and chemical reactions.
• Existing methods like density-fitted EOM-MP2 (DF-EOM-MP2) have limitations in accuracy for certain systems.
• Coupled-cluster methods, while accurate, are computationally expensive.

Purpose of the Study:

• To introduce and implement novel density-fitted equation-of-motion (DF-EOM) methods incorporating κ-regularization.
• To assess the accuracy of the new κ-DF-EOM-MP2 and κ-DF-EOM-OMP2 methods for calculating excitation energies.
• To compare the performance of these new methods against established DF-EOM-MP2, DF-EOM-OMP2, and coupled-cluster approaches.

Main Methods:

• Development and implementation of density-fitted equation-of-motion orbital-optimized second-order perturbation theory (DF-EOM-OMP2).
• Introduction of κ-regularization to DF-EOM-MP2 and DF-EOM-OMP2 methods, creating κ-DF-EOM-MP2 and κ-DF-EOM-OMP2.
• Validation against high-level EOM-CCSD(fT) calculations for excitation energies across diverse closed- and open-shell systems.

Main Results:

• The κ-regularization technique significantly improves the accuracy of excitation energy calculations, particularly for first excited states.
• κ-DF-EOM-MP2 and κ-DF-EOM-OMP2 methods demonstrate superior performance compared to their non-regularized counterparts (DF-EOM-MP2 and DF-EOM-OMP2).
• The new κ-methods achieve accuracy comparable to EOM-CCSD quality at a substantially lower computational cost.

Conclusions:

• The κ-DF-EOM-MP2 and κ-DF-EOM-OMP2 methods represent significant advancements in computational efficiency and accuracy for excitation energies.
• The κ-DF-EOM-MP2 method, in particular, shows outstanding results across various test cases.
• These κ-versions are proposed as valuable computational tools for investigating excited-state molecular properties.