Spin splittings from first-order symmetry-adapted perturbation theory without single-exchange approximation

Area of Science:

• Quantum Chemistry
• Computational Physics
• Theoretical Chemistry

Background:

• Symmetry-adapted perturbation theory (SAPT) is crucial for studying weakly interacting systems.
• Existing methods like first-order exchange energy in SF-SAPT have limitations for arbitrary spin states.
• The single-exchange approximation can be inaccurate at short intermolecular distances.

Purpose of the Study:

• To extend first-order SF-SAPT beyond the single-exchange approximation.
• To introduce and evaluate a more accurate single-spin-flip approximation for electron exchange.
• To assess the performance of these approximations on various test systems and pancake dimers.

Main Methods:

• Development of new theoretical expressions for first-order exchange energy in SF-SAPT.
• Implementation of the single-spin-flip approximation.
• Application and comparison of single-exchange and single-spin-flip approximations to model systems.

Main Results:

• The single-exchange approximation's accuracy decreases significantly at short ranges, particularly for high-spin states and small systems.
• The single-spin-flip approximation shows high accuracy across various spin states, being exact for doublet interactions.
• Both approximations yield similar accuracy for singlet-triplet splittings in pancake dimers due to even error distribution.

Conclusions:

• The single-spin-flip approximation offers a more robust and accurate treatment of electron exchange in SF-SAPT.
• This advancement is vital for reliable calculations of intermolecular interactions in diverse spin states.
• The findings provide improved tools for computational studies of molecular complexes.