The multilevel CC3 coupled cluster model.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Theoretical chemistry

Background:

• Coupled cluster (CC) methods are highly accurate for electronic structure calculations.
• Full CC3 calculations can be computationally expensive, especially for larger systems.
• Multilevel approaches offer a way to balance accuracy and computational cost.

Purpose of the Study:

• To develop an efficient implementation of a closed-shell multilevel coupled cluster method.
• To reduce the computational cost of high-accuracy electronic structure calculations for organic molecules.
• To retain the accuracy of CC3 excitation energies using a hybrid CC approach.

Main Methods:

• Implementation of a closed-shell multilevel coupled cluster method.
• Utilizing coupled cluster singles and doubles (CCSD) for the inactive orbital space.
• Employing CCSD with perturbative triples (CC3) for the active orbital space.
• Application of Cholesky orbitals for spatial localization of the active space.

Main Results:

• Achieved significant reduction in computational requirements (up to two orders of magnitude) for small organic molecules.
• Demonstrated that the method retains the accuracy of CC3 excitation energies.
• Showcased the effectiveness of Cholesky orbitals in localizing the active space and reducing cost.

Conclusions:

• The presented multilevel coupled cluster method offers a computationally efficient alternative to full CC3.
• This approach provides a practical way to obtain accurate excitation energies for small organic molecules.
• The use of Cholesky orbitals is key to the computational savings achieved.