A state-specific multi-reference coupled-cluster approach with a cost-effective treatment of connected triples: implementation to geometry optimisation

ABSTRACT

Recently, we have suggested an approximate state-specific multi-reference coupled-cluster (SS-MRCC) singles, doubles and triples method based on the CCSDT-1a+d approximation applied to the single-reference CC approach, in which the contribution of the connected triple excitations is iteratively treated. The method, abbreviated as SS-MRCCSDT-1a+d is intruder-free and fully size-extensive. It has been employed for geometry optimisations of various systems possessing quasi-degeneracy of varying degrees (like N₂H₂ and O₃) by invoking numerical gradient scheme. The method is also applied to CH₂ and square cyclobutadiene in their excited states. For all systems under study, the computed values are in good accordance with state-of-the-art theoretical estimates indicating that the method might be a promising candidate for an accurate treatment of geometrical parameters of states plagued by electronic degeneracy in a computationally tractable manner.

GRAPHICAL ABSTRACT

Keywords:

• State-specific multi-reference coupled cluster
• iterative triples correction
• gradient scheme
• geometry optimisation
• ground and excited states

Acknowledgments

D. Banerjee wishes to sincerely acknowledge the inspiration obtained from her colleagues at the Department of Physics, Shibpur Dinobundhoo Institution (College). U. Sinha Mahapatra thanks the Department of Chemistry, Maulana Azad College, for various support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work has been supported by the Department of Science and Technology, India [grant number SR/S1/PC-61/2009].