From complete to selected model spaces in determinant-based multi-reference second-order perturbation treatments.

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• The paper addresses limitations in existing determinant-based second-order multi-reference perturbative formalisms.
• Previous methods required a complete active space for size consistency, a significant restriction.

Purpose of the Study:

• To reformulate and improve a determinant-based second-order multi-reference perturbative formalism.
• To extend the applicability of size consistency beyond complete active spaces.

Main Methods:

• A simple modification of energy denominators to include interactions between model space determinants.
• Investigating conditions for size consistency with incomplete model spaces.
• Illustrating the method's robustness with model problems.

Main Results:

• The reformulated method achieves size consistency without requiring a complete active space.
• Size consistency is attainable from truncated complete active spaces and Singles and Doubles Configuration Interactions with coupled electron pair approximation.
• The method allows flexible revision of the model-space wave function component.

Conclusions:

• The improved formalism offers enhanced size consistency for multi-reference electronic structure calculations.
• This advancement broadens the applicability of accurate quantum chemical methods.
• The flexibility in treating dynamic correlation effects provides deeper physical insights.