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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

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Published on: April 8, 2020

An explicitly spin-free compact open-shell coupled cluster theory using a multireference combinatoric exponential

Dipayan Datta¹, Debashis Mukherjee

¹Department of Physical Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India.

The Journal of Chemical Physics

|August 7, 2009

Summary

We developed a new spin-free multireference coupled cluster (CC) method for open-shell systems. This approach accurately calculates state energies for radicals and molecular ions, showing high efficacy compared to other methods and experiments.

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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

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Area of Science:

Quantum Chemistry
Computational Chemistry
Theoretical Chemistry

Background:

Accurate calculation of electronic structure for open-shell systems is crucial in chemistry.
Existing methods often struggle with the complexity of open-shell electronic configurations.

Purpose of the Study:

To present a novel spin-free, state-universal, multireference coupled cluster (CC) formalism.
To enable computation of state energies for simple open-shell systems like doublets and biradicals.

Main Methods:

Development of a spin-free compact multireference CC formalism.
Definition of cluster operators (T(mu), S(e)(mu), S(re)(mu)) based on spin-free unitary generators.
Introduction of a new wave operator ansatz with automorphic factors to avoid redundant excitations.

Main Results:

The formalism provides a comprehensive account of spin-free cluster operators.
The derived CC equations feature direct and normalization terms.
Applications to doublet radicals, cations, and ionization energies yielded highly encouraging results.

Conclusions:

The developed spin-free multireference CC method is effective for open-shell systems.
Results align well with other CC theories, full CI benchmarks, and experimental data.
This formalism offers a powerful tool for studying open-shell molecular systems.