Multireference Brillouin-Wigner coupled cluster method with singles, doubles, and triples: efficient implementation...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Molecular Modeling

Background:

• Accurate prediction of molecular electronic structure is crucial in chemistry.
• Approximate methods are necessary for larger systems but require validation.
• Multireference coupled cluster methods offer high accuracy but are computationally demanding.

Purpose of the Study:

• To develop an efficient implementation of the multireference Brillouin-Wigner coupled cluster (MRBWCC) method.
• To provide a benchmark for assessing the accuracy of approximate computational chemistry methods.
• To validate the MRBWCC implementation against experimental data.

Main Methods:

• Full iterative treatment of connected singles, doubles, and triples within the MRBWCC framework.
• Application of the method to the ground and low-lying excited states of the oxygen molecule (O2).
• Calculation of the singlet-triplet gap for methylene (CH2).

Main Results:

• An efficient MRBWCC implementation was successfully developed.
• The method's computational cost is high, limiting its use to smaller systems.
• Calculations for O2 and CH2 showed good agreement with experimental data.
• The MRBWCC method serves as a reliable benchmark for approximate techniques.

Conclusions:

• The developed MRBWCC method is a valuable tool for benchmarking approximate computational chemistry approaches.
• The implementation provides accurate results for challenging electronic structure problems.
• Further applications are feasible for smaller molecules and specific electronic structure investigations.