Frozen Natural Orbitals-Based Coupled-Cluster Singles, Doubles, and (full) Triples - A Computational Study

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• The Frozen Natural Orbitals (FNO) approach provides a computationally efficient method for calculating molecular energies using Coupled Cluster (CC) methods.
• Existing FNO implementations are limited to CC Singles and Doubles (CCSD) and Equation-of-Motion CCSD (EOM-CCSD) levels of theory.

Purpose of the Study:

• To extend the Frozen Natural Orbitals (FNO) approach to the Coupled Cluster with Singles, Doubles, and Triples (CCSDT) level of theory.
• To implement and evaluate the performance of FNO-CCSDT within the Q-CHEM quantum chemistry package.
• To explore the accuracy and efficiency of FNO-CCSDT for various molecular properties and spectroscopic parameters.

Main Methods:

• Implementation of the Frozen Natural Orbitals (FNO) approach for Coupled Cluster with Singles, Doubles, and Triples (CCSDT) calculations.
• Utilized both Double Precision (DP) and Single Precision (SP) algorithms for FNO-CCSDT.
• Extrapolation of FNO-CCSDT results to obtain the XFNO-CCSDT approach for enhanced accuracy.
• Calculated total energies, adiabatic and vertical triplet-singlet gaps, and estimated spectroscopic parameters (force constants, vibrational frequencies).

Main Results:

• Demonstrated the feasibility and efficiency of the FNO-CCSDT method for accurate energy computations.
• Showcased the performance of FNO-CCSDT compared to conventional CCSDT using water as a test system.
• Presented results for molecular energies, triplet-singlet gaps, and bond-stretching trends, highlighting the accuracy of the extrapolated XFNO-CCSDT approach.
• Successfully applied FNO-CCSDT for the numerical estimation of spectroscopic parameters in diatomic molecules.

Conclusions:

• The FNO approach is successfully extended to the CCSDT level, providing a valuable tool for accurate and cost-effective quantum chemical calculations.
• The FNO-CCSDT method, including its extrapolated XFNO-CCSDT variant, offers a significant improvement in computational efficiency without compromising accuracy.
• This work paves the way for applying high-level coupled cluster theory to larger and more complex molecular systems.