Analytic gradients for multiconfiguration pair-density functional theory with density fitting: Development and...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• Traditional methods involving four-index electron repulsion integrals are computationally expensive, limiting the size of treatable systems.
• Multireference treatments are necessary for systems with strong electron correlation but are often computationally prohibitive.

Purpose of the Study:

• To derive and implement density fitting for multiconfiguration pair-density functional theory (MCPDFT).
• To evaluate the accuracy and efficiency of MCPDFT with density fitting for energy and gradient calculations.
• To assess the impact of reduced grid sizes on computational cost and optimized geometries.

Main Methods:

• Derivation of energies and analytic gradients for MCPDFT using density fitting.
• Application of the method to six chemical systems.
• Comparison of results with methods using exact electron repulsion integrals and complete active space second-order perturbation theory (CASPT2).
• Investigation of reduced grid size effects.

Main Results:

• Density fitting significantly reduces computational cost for MCPDFT calculations.
• The approach achieves a substantial increase in computational speed with negligible loss in accuracy.
• Optimized geometries and gradient values show little sensitivity to reduced grid sizes.
• MCPDFT with density fitting provides an efficient route to accurate multireference geometry optimization.

Conclusions:

• Density fitting is an effective approximation for accelerating MCPDFT calculations.
• The developed method enables efficient and accurate geometry optimization of large molecular systems.
• This work provides a valuable tool for computational chemists studying complex electronic structures.