Quasi-degenerate perturbation theory with general multiconfiguration self-consistent field reference functions

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Quasi-degenerate perturbation theory (QDPT) is a powerful method for electronic structure calculations.
• Existing QDPT methods often rely on complete active space (CAS) self-consistent field (SCF) reference functions.
• Extending QDPT to more general multiconfiguration (MC) SCF reference functions is crucial for broader applicability.

Purpose of the Study:

• To generalize quasi-degenerate perturbation theory (QDPT) for multiconfiguration (MC) self-consistent field (SCF) reference functions.
• To develop a computational scheme combining diagrammatic and sum-over-states approaches for QDPT.
• To validate the extended QDPT method through calculations on small molecules.

Main Methods:

• Extension of QDPT to general MC-SCF reference functions.
• A hybrid computational scheme utilizing diagrammatic and sum-over-states (SOS) approaches.
• Calculation of the second-order effective Hamiltonian using both approaches for different types of configurations.

Main Results:

• The generalized QDPT method was successfully applied to H2O, LiF, and H2CO.
• Calculated excitation energies and potential energy curves show excellent agreement with CAS-SCF reference QDPT results.
• Deviations from experimental values were minimal, indicating high accuracy.

Conclusions:

• The developed QDPT method with general MC-SCF references is accurate and reliable.
• This extension broadens the applicability of QDPT in computational chemistry.
• The method provides a valuable tool for studying molecular electronic structures and properties.