Hybrid coupled cluster methods: combining active space coupled cluster methods with coupled cluster singles, doubles,...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Molecular Modeling

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Existing methods like CCSD(T) may struggle with molecules exhibiting significant multireference character.
• Development of advanced CC methods is crucial for tackling complex chemical systems.

Purpose of the Study:

• To develop and implement novel hybrid coupled cluster (CC) methods.
• To address the electronic structure of molecules with significant multireference character.
• To improve the accuracy of calculating reaction barriers and bond dissociation processes.

Main Methods:

• Developed CCSD(T)-h and CCSD(T)q-h hybrid CC methods.
• Utilized both non-canonical and canonical molecular orbitals (MOs).
• Proposed a practical procedure for dividing canonical MOs into active and inactive subsets.

Main Results:

• CCSD(T)-h with canonical MOs accurately reproduces results obtained with non-canonical MOs.
• CCSD(T)-h significantly improves upon CCSD(T) for describing reaction barriers in three-atom exchange reactions.
• CCSD(T)q-h approximates CCSDTQ for bond-breaking processes in F2 and H2O.

Conclusions:

• The new hybrid CC methods offer enhanced accuracy for challenging molecular systems.
• CCSD(T)-h and CCSD(T)q-h provide reliable computational tools for electronic structure studies.
• These methods advance the capability of computational chemistry in predicting chemical reaction dynamics.