Relaxed active space: fixing tailored-CC with high order coupled cluster. II

Area of Science:

• Computational chemistry
• Quantum chemistry
• Electronic structure theory

Background:

• Coupled-cluster methods, particularly those including higher-connected cluster operators like CCSDT, face significant computational cost increases with system size and basis set.
• Treating larger chemical systems with high accuracy often requires approximations or partitioning strategies like active space selection.
• Manual selection of active spaces can be subjective and may not guarantee sufficient accuracy for challenging electronic structures.

Purpose of the Study:

• To develop an automated, statistically driven scheme for generating active spaces in coupled-cluster calculations.
• To enable the application of high-level coupled-cluster methods, specifically CCSDT, to larger and more complex molecular systems.
• To remove the user's burden of subjectively judging the adequacy of a chosen active space.

Main Methods:

• An automated scheme based on unambiguous statistical criteria was implemented to define the active space.
• The selected active space allows for treatment at the coupled-cluster with singles, doubles, and triples (CCSDT) level of theory.
• The methodology was applied to demanding test cases, including twisted ethylene and transition states for bicyclo[1,1,0]butane isomerization.

Main Results:

• The automated scheme successfully generated active spaces suitable for CCSDT calculations.
• The method allowed for accurate treatment of systems with complex electronic structures, such as biradicals.
• Quantitative results for geometries and energies were obtained for twisted ethylene and bicyclo[1,1,0]butane isomerization transition states.

Conclusions:

• The developed automated active space generation is a practical solution for high-accuracy coupled-cluster calculations on larger systems.
• This statistically guided approach enhances the reliability and objectivity of computational chemistry studies.
• The method is particularly valuable for systems like biradicals that necessitate high levels of electronic structure theory.