FCIQMC-Tailored Distinguishable Cluster Approach.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• Traditional methods like coupled cluster and configuration interaction face challenges with large systems and basis set convergence.
• The distinguishable cluster method offers a promising route for accurate calculations.

Purpose of the Study:

• To develop and evaluate a new tailored approach for the distinguishable cluster method.
• To improve the accuracy and efficiency of quantum chemical calculations.
• To introduce a novel active space definition strategy.

Main Methods:

• Application of a tailored approach to the distinguishable cluster method.
• Integration with a stochastic Full Configuration Interaction Quantum Monte Carlo (FCIQMC) solver.
• Introduction of an F12 correction for enhanced basis set convergence.
• Evaluation of a black-box active space definition using natural orbitals from distinguishable clusters.

Main Results:

• The new tailored distinguishable cluster-FCIQMC method demonstrates superior accuracy compared to tailored coupled cluster and pure distinguishable cluster methods.
• The F12 correction significantly accelerates basis set convergence.
• The natural orbital-based active space definition proves to be a practical alternative to CASSCF.

Conclusions:

• The developed tailored distinguishable cluster-FCIQMC approach represents a significant advancement in computational quantum chemistry.
• The F12 correction is essential for achieving efficient and accurate results.
• The new active space definition method offers a user-friendly and effective approach for defining active spaces.