Explicitly correlated multireference configuration interaction: MRCI-F12.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multireference configuration interaction (MRCI) is a standard method for electronic structure calculations.
• Traditional MRCI methods suffer from slow basis set convergence, requiring large basis sets for accurate results.
• Explicitly correlated methods can accelerate basis set convergence.

Purpose of the Study:

• To develop an efficient explicitly correlated variant of the internally contracted multireference configuration interaction method (MRCI-F12).
• To assess the performance of the MRCI-F12 method in terms of accuracy and computational cost.
• To demonstrate the applicability of MRCI-F12 to challenging chemical problems.

Main Methods:

• Development of an internally contracted multireference configuration interaction method incorporating explicit dependence on the electron-electron distance (MRCI-F12).
• Utilization of the F12b approximation for computational efficiency.
• Application of MRCI-F12 to calculate the singlet-triplet separation of methylene, dissociation energy of ozone, properties of diatomic molecules, and the reaction barrier/exothermicity of F + H(2).

Main Results:

• The MRCI-F12 method exhibits significantly improved basis set convergence compared to traditional MRCI.
• The method achieves near complete basis set MRCI accuracy even with moderately sized basis sets.
• Quantitative agreement with experimental data was observed for all tested systems.
• An explicitly correlated multireference averaged coupled pair functional method (MRACPF-F12) was also implemented.

Conclusions:

• The MRCI-F12 method provides a computationally efficient and accurate approach for electronic structure calculations.
• It offers a significant improvement over standard MRCI methods, particularly in basis set convergence.
• MRCI-F12 is a valuable tool for studying various chemical phenomena with high fidelity.