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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

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Published on: April 8, 2020

Multireference equation-of-motion coupled cluster theory.

Dipayan Datta¹, Marcel Nooijen

¹Institut für Physikalische Chemie, Johannes Gutenberg-Universität Mainz, D-55099 Mainz, Germany. datta@uni-mainz.de

The Journal of Chemical Physics

|December 5, 2012

Summary

A new multireference coupled cluster theory method accurately calculates multiple electronic states. This approach optimizes dynamical correlation for a parent state and then determines state-specific effects for accurate results.

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Area of Science:

Quantum Chemistry
Computational Physics
Theoretical Chemistry

Background:

Traditional equation-of-motion coupled cluster (EOM-CC) methods are limited in describing multiple electronic states simultaneously.
Accurate calculation of electronic states is crucial for understanding molecular properties and spectra.
Multireference approaches are needed for systems with strong electron correlation or near-degeneracies.

Purpose of the Study:

To develop a generalized equation-of-motion coupled cluster theory applicable to multiple electronic states.
To enable the calculation of electronic states that share similar active spaces.
To improve the accuracy and efficiency of calculating excitation spectra and electronic state properties.

Main Methods:

A multireference parent state is used as a reference for the coupled cluster ansatz.
Internally contracted multireference coupled cluster theory optimizes dominant dynamical correlation for the parent state.
An uncontracted diagonalization of the transformed Hamiltonian in a compact multireference configuration interaction space accounts for remaining correlation and orbital relaxation.

Main Results:

The proposed method successfully calculates multiple electronic states by optimizing a parent state and then determining state-specific correlations.
The use of many-body residuals derived from the transformed Hamiltonian ensures non-singular equations for amplitude determination.
Preliminary calculations on C2, O2, and transition metals (Fe, Cr, Mn) show the method's potential for diverse systems, including hundreds of excited states.

Conclusions:

The generalized equation-of-motion coupled cluster theory provides a robust framework for calculating multiple electronic states.
The method ensures the transferability of dynamical correlation from the parent state to target states.
This approach offers a promising avenue for accurate electronic structure calculations across various chemical and physical systems.