Implementation of the CCSD(T)-F12 method using cusp conditions.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster methods are essential for accurate electronic structure calculations.
• Explicitly correlated methods, like CCSD(T)-F12, improve convergence and efficiency.
• Cusp conditions provide valuable constraints for electronic wavefunctions.

Purpose of the Study:

• To implement the explicitly-correlated coupled-cluster singles and doubles with perturbative triples (CCSD(T)-F12) method using cusp conditions.
• To assess the accuracy of the cusp-condition CCSD(T)-F12 method for molecular energies and reaction enthalpies.
• To develop an extended ansatz capable of satisfying singlet and triplet cusp conditions in open-shell systems.

Main Methods:

• Implementation of the CCSD(T)-F12 method incorporating cusp conditions.
• Numerical testing on a set of 16 molecules to evaluate correlation energies.
• Benchmark calculations on 13 chemical reactions to compare with experimental enthalpies.
• Development of an extended unitary-invariant ansatz for open-shell systems.

Main Results:

• Correlation energies obtained using the cusp-condition CCSD(T)-F12 method agreed within 1 mE(h) with fully-optimized CCSD(T)-F12 results.
• Benchmark calculations reproduced experimental enthalpies of chemical reactions within 2 kJ mol(-1).
• Demonstrated that regular unitary-invariant ansatz fails to satisfy singlet and triplet cusp conditions for open-shell systems.

Conclusions:

• The cusp-condition CCSD(T)-F12 method provides a computationally efficient and accurate approach for electronic structure calculations.
• The developed extended ansatz successfully handles singlet and triplet cusp conditions in open-shell situations.
• This work advances the accuracy and applicability of explicitly correlated coupled-cluster methods in computational chemistry.