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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
Published on: April 8, 2020
Coupling term derivation and general implementation of state-specific multireference coupled cluster theories
Francesco A Evangelista1, Wesley D Allen, Henry F Schaefer
1Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA. frank@ccc.uga.edu
The Journal of Chemical Physics
|July 21, 2007
Summary
New multireference coupled cluster (MRCC) theories and code (PSIMRCC) offer accurate predictions for molecular properties. The Mukherjee method (MkCCSD) outperforms Brillouin-Wigner theory (BWCCSD) for challenging chemical systems.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Theoretical Chemistry
Background:
- State-specific multireference coupled cluster (MRCC) theories require renormalization terms.
- Existing methods face challenges in accurately describing complex electronic structures.
Purpose of the Study:
- Derive simple closed-form expressions for "same vacuum" renormalization terms in MRCC theories.
- Develop and test production-level code for state-specific multireference coupled cluster calculations.
- Compare the performance of Mukherjee (MkCCSD) and Brillouin-Wigner (BWCCSD) methods.
Main Methods:
- Derived "same vacuum" renormalization terms for MRCC theories up to triple excitations.
- Developed the PSIMRCC code for state-specific Mukherjee and Brillouin-Wigner coupled cluster calculations.
- Applied MkCCSD and BWCCSD methods to F(2) dissociation, ozone structure/vibrations, and benzyne singlet-triplet splittings.
Main Results:
- MkCCSD and BWCCSD methods were implemented and tested on classic multireference problems.
- MkCCSD demonstrated superior accuracy compared to BWCCSD for energies, structures, and vibrational frequencies.
- Localized molecular orbitals enhanced MkCCSD accuracy for F(2) dissociation.
- MkCCSD improved ozone calculations but requires triple excitations for the antisymmetric stretching frequency.
- Preliminary MkCCSD results for benzyne singlet-triplet splittings closely matched experimental data.
Conclusions:
- The derived renormalization terms simplify MRCC calculations.
- The PSIMRCC code provides a robust tool for advanced quantum chemistry studies.
- MkCCSD is a promising method for accurately describing challenging chemical systems, outperforming BWCCSD.
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