Spin-flip equation-of-motion coupled cluster method with singles, doubles and (full) triples: computational...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate calculation of electronic structure is crucial for understanding molecular properties.
• Existing methods like SF-EOM-CCSD may lack quantitative accuracy for certain systems.
• Inclusion of triple excitations is known to improve accuracy in coupled-cluster methods.

Purpose of the Study:

• To computationally implement the spin-flip equation-of-motion coupled-cluster method with singles, doubles, and full triples (SF-EOM-CCSDT).
• To assess the accuracy and applicability of the SF-EOM-CCSDT method.
• To investigate the performance of both double precision (DP) and single precision (SP) algorithms.

Main Methods:

• Implementation of SF-EOM-CCSDT within the Q-CHEM computational chemistry package.
• Comparison of SF-EOM-CCSDT results with benchmark methods: Full Configuration Interaction (FCI) and Complete Active Space Self-Consistent Field second-order (CASSCF-SO) CI.
• Application to study adiabatic energy gaps in diradicals (CH2, NH2+), vertical excitation energies (CH radicals), and bond dissociation (HF).

Main Results:

• The inclusion of triples significantly enhances the quantitative accuracy of SF-EOM-CCSD, providing correct qualitative trends for energy gaps between degenerate states.
• SF-EOM-CCSDT results show good agreement with FCI and CASSCF-SO benchmarks for the studied systems.
• Single precision (SP) implementation of SF-EOM-CCSDT yields accurate energies and energy gaps with reduced computational cost compared to double precision (DP).

Conclusions:

• SF-EOM-CCSDT is a highly accurate method for calculating electronic excitation energies and energy gaps.
• The SP implementation of SF-EOM-CCSDT offers a computationally efficient approach, expanding its applicability to larger molecular systems.
• This work provides a valuable tool for theoretical chemists studying complex electronic structures and reaction dynamics.