Benchmark studies of variational, unitary and extended coupled cluster methods

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled cluster theory is a standard method for electronic structure calculations.
• Traditional coupled cluster methods can struggle with strong static correlation.
• Accurate prediction of molecular properties is crucial in chemistry.

Purpose of the Study:

• To compare the performance of standard, variational, extended, and unitary coupled cluster methods.
• To evaluate these methods for systems with varying degrees of static correlation.
• To assess the inclusion of quadruple excitations in coupled cluster calculations.

Main Methods:

• Comparative benchmark calculations using standard, variational, extended, and unitary coupled cluster theories.
• Inclusion of cluster operators up to quadruple excitations.
• Application to small molecular systems like HF, N(2), and CN.

Main Results:

• Differences between methods are negligible when static correlation is weak.
• Variational coupled cluster theory demonstrates superior robustness in the presence of strong static correlation.
• Errors are significantly reduced with the variational approach across all levels of truncation.

Conclusions:

• Variational coupled cluster theory provides a more reliable framework than traditional methods for systems with strong static correlation.
• This method serves as a foundation for single-determinant reference methods applicable beyond their usual limits.
• The robustness of the variational approach is consistent even with truncated cluster operators.