The Auxiliary-Field Quantum Monte Carlo Method with Seniority-Zero Trial Wave Function.

Area of Science:

• Quantum Chemistry
• Computational Physics
• Materials Science

Background:

• Accurate quantum chemical calculations are crucial for understanding chemical systems.
• Traditional methods like Complete Active Space (CAS) Configuration Interaction (CI) are computationally expensive.
• Developing efficient methods to capture both static and dynamic electron correlation is a key challenge.

Purpose of the Study:

• To introduce and evaluate a novel approach combining Doubly Occupied Configuration Interaction (DOCI) with phaseless Auxiliary-Field Quantum Monte Carlo (ph-AFQMC).
• To assess the performance of this method, including an orbital-optimized variant (OO-DOCI), for various chemical systems.
• To explore the potential of DOCI-based ph-AFQMC as a computationally efficient alternative to CAS methods for multireference calculations.

Main Methods:

• Utilizing the DOCI wave function as a trial wave function within the ph-AFQMC framework.
• Employing an orbital-optimized DOCI (OO-DOCI) to enhance accuracy.
• Testing the method on systems like O-H bond breaking in water, polymer additives, carbon dimer, and hydrogen systems.

Main Results:

• The OO-DOCI-AFQMC method achieved high accuracy comparable to CAS-based ph-AFQMC for systems with moderate correlation.
• Performance closely matched or exceeded coupled-cluster methods for specific cases.
• Accuracy decreased for strongly correlated systems, indicating limitations of the seniority-zero space for such scenarios.

Conclusions:

• DOCI- and OO-DOCI-based ph-AFQMC present a computationally tractable approach for accurate multireference calculations.
• The method effectively captures static and dynamic correlation for many systems, reducing the cost associated with CAS methods.
• Further development is needed for strongly correlated systems, potentially involving trial wave functions in an extended space.