Renormalized Internally Contracted Multireference Coupled Cluster with Perturbative Triples

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The driven similarity renormalization group (DSRG) offers a unitary multireference coupled cluster theory.
• Numerical instabilities can arise in coupled cluster methods.
• Accurate calculations for strongly correlated systems remain a challenge.

Purpose of the Study:

• To develop a novel computational method for accurate electronic structure calculations.
• To extend the DSRG approach to nonunitary transformations for broader applicability.
• To improve the efficiency of multireference coupled cluster calculations.

Main Methods:

• Combined internally contracted multireference coupled cluster (ic-MRCC) with the driven similarity renormalization group (DSRG).
• Adapted the unitary flow equation approach for nonunitary transformations, creating the renormalized ic-MRCC (ric-MRCC) method.
• Introduced approximations to the Baker-Campbell-Hausdorff expansion and approximate perturbative triples for ric-MRCCSD[T].

Main Results:

• Demonstrated the accuracy of ric-MRCC methods for potential energy curves of H8, F2, H2O, N2, and Cr2.
• Achieved accuracy comparable to advanced multireference methods.
• ric-MRCCSD and ric-MRCCSD[T] matched CCSD(T) and full configuration interaction results for spectroscopic constants and energies.

Conclusions:

• The developed renormalized ic-MRCC (ric-MRCC) method provides a computationally efficient and accurate approach for electronic structure calculations.
• This method effectively addresses challenges in multireference systems.
• The findings pave the way for more reliable predictions in quantum chemistry.