First-Order Symmetry-Adapted Perturbation Theory with Double Exchange for Multireference Systems

Area of Science:

• Quantum chemistry
• Computational chemistry
• Theoretical chemistry

Background:

• Symmetry-adapted perturbation theory (SAPT) is crucial for calculating intermolecular interaction energies.
• First-order SAPT (SAPT(MC)) has been extended to include electron correlation effects.
• Accurate calculation of exchange energies, especially double-exchange effects, remains a challenge.

Purpose of the Study:

• To extend first-order multiconfigurational symmetry-adapted perturbation theory (SAPT(MC)) to incorporate double-exchange effects.
• To develop density-matrix-based expressions for first-order exchange energy.
• To evaluate the performance of these new methods for molecular interactions.

Main Methods:

• Derivation of density-matrix-based expressions for first-order exchange energy.
• Application of double-exchange approximation to strongly orthogonal geminal wave functions.
• Development of an approximate method for double-exchange energy with complete active space (CAS) wave functions.

Main Results:

• Successful extension of SAPT(MC) to account for double-exchange effects.
• Numerical demonstration using strongly orthogonal geminal wave functions.
• Analysis of approximate double-exchange energy evaluation with CAS wave functions on model dimers.

Conclusions:

• The developed methods provide a more accurate treatment of intermolecular interactions by including double-exchange effects.
• The proposed approximate method offers a practical approach for evaluating double-exchange energy in CAS wave functions.
• The study advances the capability of SAPT(MC) for analyzing ground and excited states of molecular systems.