Analytic gradients for restricted active space second-order perturbation theory (RASPT2)

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Computational cost in multireference perturbation theory is limited by active space size.
• Analytic derivatives are crucial for efficient electronic structure calculations.

Purpose of the Study:

• To develop and implement analytic gradients for single-state restricted active space second-order perturbation theory (RASPT2).
• To overcome previous limitations on active space size in computational chemistry.

Main Methods:

• Implementation of analytic gradients for RASPT2 and complete active space second-order perturbation theory (CASPT2) in OpenMolcas.
• Utilized the Lagrangian or Z-vector method, similar to CASPT2 implementations.

Main Results:

• Enabled the use of restricted active spaces with up to 20 electrons in 20 orbitals.
• Demonstrated feasibility for geometry optimizations with significantly larger active spaces.

Conclusions:

• The developed RASPT2 analytic gradients significantly reduce computational cost.
• This method expands the applicability of high-level theoretical methods to larger chemical systems.