The externally corrected coupled cluster approach with four- and five-body clusters from the CASSCF wave function

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate prediction of molecular properties is crucial in chemistry.
• Coupled cluster methods are powerful but computationally expensive.
• Complete active space self-consistent field (CASSCF) provides a good reference but lacks high-level correlation.

Purpose of the Study:

• Introduce a novel externally corrected coupled cluster method (ecCCSDt-CASSCF).
• Improve the accuracy of theoretical calculations for challenging chemical systems.
• Investigate the performance of ecCCSDt-CASSCF compared to existing methods.

Main Methods:

• Developed an externally corrected CCSDt (coupled cluster with singles, doubles, and active triples) approach.
• Integrated four- and five-body cluster amplitudes from CASSCF into CCSDt equations.
• Ensured size-extensivity by using a size-extensive CASSCF reference function.

Main Results:

• The ecCCSDt-CASSCF method is size-extensive and computationally efficient.
• Applied to study H2O dissociation, diatomic molecules (F2+, O2+, Be2, NiC), and reaction barriers.
• Demonstrated superior accuracy compared to CASPT2 and CCSDT methods in most cases.

Conclusions:

• The ecCCSDt-CASSCF method provides a robust and accurate approach for electronic structure calculations.
• This method offers a promising balance between accuracy and computational cost.
• It advances the capability for studying complex chemical phenomena.