Regularized CASPT2: an Intruder-State-Free Approach

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The intruder-state problem (ISP) is a known issue in multi-reference electronic structure methods like CASPT2.
• Existing methods to address ISP include real and imaginary level shifts, which have limitations.

Purpose of the Study:

• To present and evaluate a novel σ-regularization approach for resolving the ISP in CASPT2.
• To compare the efficacy and parameter sensitivity of σ-CASPT2 against established level shift techniques.

Main Methods:

• Development of the σ-CASPT2 method based on σ-regularization.
• Theoretical analysis of the σ-CASPT2 method.
• Systematic computational calculations comparing σ-CASPT2 with real and imaginary level shifts.

Main Results:

• The σ-CASPT2 method effectively removes the intruder-state problem.
• Different versions, σ¹-CASPT2 and σ²-CASPT2, demonstrate varied application potentials.
• The real level shift technique is found to be unsuitable for general ISP mitigation.

Conclusions:

• σ-CASPT2 offers a robust and effective solution to the intruder-state problem in CASPT2.
• The choice between σ¹-CASPT2 and σ²-CASPT2 depends on specific application requirements.
• This work highlights the limitations of real level shifts for addressing ISP.