Quasi-degenerate second-order perturbation theory for occupation restricted multiple active space self-consistent...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate quantum chemical calculations are crucial for understanding molecular behavior.
• Approximating complex wavefunctions, such as complete active space self-consistent field (CASSCF), is computationally demanding.
• Occupation-restricted multiple active space (ORMAS) offers a way to simplify these calculations by using a subset of configurations.

Purpose of the Study:

• To present a novel multi-configuration quasi-degenerate second-order perturbation method.
• To introduce the occupation-restricted multiple active space perturbation theory (ORMAS-PT) based on ORMAS.
• To demonstrate the performance and applicability of the ORMAS-PT method.

Main Methods:

• Development of a multi-configuration quasi-degenerate second-order perturbation theory.
• Utilizing the occupation restricted multiple active space (ORMAS) wavefunction as a reference.
• Application of multi-reference Møller-Plesset formalism to correct the ORMAS reference energy.
• Implementation using a direct configuration interaction (CI) methodology.

Main Results:

• The ORMAS-PT method provides an approximation to CASSCF wavefunctions.
• The method corrects the ORMAS reference energy using perturbation theory.
• Computational tests demonstrate the effectiveness and performance of the ORMAS-PT approach.
• The direct CI scheme facilitates efficient computation.

Conclusions:

• ORMAS-PT offers a computationally feasible approach to approximate accurate wavefunctions.
• The method shows promise for improving the efficiency of quantum chemical calculations.
• Further testing and application of ORMAS-PT are warranted for various chemical systems.