Communication: Second-order multireference perturbation theory with explicit correlation: CASPT2-F12.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• Basis set incompleteness can limit the accuracy of traditional quantum chemical methods.
• Explicitly correlated methods aim to improve convergence with respect to basis set size.

Purpose of the Study:

• To develop and present an explicitly correlated complete active space second-order perturbation (CASPT2-F12) method.
• To enhance the convergence of CASPT2 energies and properties with respect to basis set size.
• To assess the computational efficiency and accuracy of the new method.

Main Methods:

• Implementation of an explicitly correlated CASPT2 method incorporating a Slater-type geminal.
• Internal contraction of explicitly correlated terms in the wave function.
• Application of the CASPT2-F12 method to benchmark chemical systems.

Main Results:

• The CASPT2-F12 method significantly accelerates the convergence of energies and properties with increasing basis set size.
• The computational overhead for the F12 correction is demonstrated to be small.
• Accurate results were obtained for the singlet-triplet splitting of methylene, ozone dissociation energy, and pyrrole excited states.

Conclusions:

• The presented CASPT2-F12 method offers a computationally efficient way to achieve high accuracy in electronic structure calculations.
• This method effectively addresses the basis set convergence issues inherent in conventional CASPT2.
• CASPT2-F12 is a promising tool for studying various chemical phenomena requiring precise electronic structure data.