Configuration-driven unitary group approach for generalized Van Vleck variant multireference perturbation theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multireference perturbation theory is crucial for describing complex electronic structures.
• Previous implementations, like Table-CI, faced computational limitations.
• Generalized van Vleck perturbation theory (GVVPT) offers a framework for these calculations.

Purpose of the Study:

• To develop and implement an efficient, configuration-driven algorithm for GVVPT.
• To improve the computational speed of GVVPT calculations.
• To validate the new algorithm's applicability and accuracy.

Main Methods:

• Developed a new algorithm utilizing the unitary group approach (UGA).
• Implemented the algorithm for both GVVPT2 and GVVPT3 approximations.
• Applied the method to model problems and a chemically relevant system (cyclobutadiene).

Main Results:

• Achieved computational speed improvements of 1-2 orders of magnitude compared to previous methods.
• Demonstrated applicability to both second- and third-order GVVPT.
• Obtained accurate geometric parameters and automerization barrier height for cyclobutadiene, consistent with high-accuracy results, using over 2.3 billion configuration state functions (CSFs).

Conclusions:

• The new UGA-based algorithm offers a significant computational advantage for GVVPT.
• The method provides accurate results for complex chemical systems.
• This advancement enables more efficient and reliable theoretical studies in quantum chemistry.