Explicitly correlated coupled-cluster theory with Brueckner orbitals

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Brueckner orbitals are ideal for F12 explicitly correlated coupled-cluster (CC) calculations.
• Existing methods can be computationally expensive due to repeated F12 integral re-evaluation.

Purpose of the Study:

• Introduce a novel Brueckner coupled-cluster doubles with perturbative triples (BCCD(T)(F12*)) method.
• Generalize F12 theory for non-Hartree-Fock references and relativistic effects.
• Assess the performance of the new Brueckner F12 method.

Main Methods:

• Developed BCCD(T)(F12*) to bypass costly F12 integral re-evaluation during orbital optimization.
• Incorporated a new basis set correction for the Brueckner reference energy.
• Extended F12 theory to arbitrary non-Hartree-Fock references and scalar relativistic Fock operators.

Main Results:

• The BCCD(T)(F12*) method demonstrates systematic improvement over CCSD(T)(F12*) for electron affinities.
• Performance was evaluated on 50 open- and closed-shell reactions and transition metal atom ionization potentials/electron affinities.
• Benchmark basis set limit coupled-cluster singles, doubles and perturbative triples (CCSD(T)) and BCCD(T) values were computed.

Conclusions:

• BCCD(T)(F12*) provides a more accurate and efficient approach for electronic structure calculations.
• The method is particularly advantageous for systems with significant orbital relaxation effects, such as electron affinities.
• The generalization of F12 theory expands its applicability to a wider range of chemical systems.