Exploring the spectroscopic constants of 2Πu, 2Σu+ (main and a satellite) and 2Πg states of N2+ ion using the Fock-space coupled-cluster with eigenvalue independent partitioning

Abstract

The Fock-space coupled-cluster (FSCC) method appears as a robust computational scheme if reformulated within the eigenvalue independent partitioning framework (EIP-FSCC) which efficiently deals with the intruder states that cause convergence problems in its standard effective Hamiltonian format. Within such a reformulation not only the so-called main (principal) root of the effective Hamiltonian version are attainable, but also the alternative satellite roots appear from the higher-sectors of the corresponding matrix (that is properly constructed) upon its direct diagonalization. Selective spectroscopic constants of ${}^2{\mathrm{\Pi }}_u$, ${}^2{\mathrm{\Sigma }}_u^{+}$ (main and satellite), and ${}^2{\mathrm{\Pi }}_g$ states of N${}_2^{+}$ ion have here been estimated using the EIP-FSCC by exploiting various truncation schemes of cluster operators (EIP-FSCCSD and EIP-FSCCSDτ). EIP-FSCCSDτ includes the set of full connected triple excitation operators for the ionized states where as perturbative is considered by CCSDT1-a approximation. This work indicates that the EIP-FSCCSDτ, compared to EIP-FSCCSD scheme, is capable of providing nice agreement with experiment provided an adequate basis set is used.

GRAPHICAL ABSTRACT

Keywords:

• Fock-space
• coupled cluster
• eigenvalue independent partitioning
• main and satellite IPs

Acknowledgments

It is a great pleasure to present this work in this volume that is dedicated to Professor Jürgen Gauss. .His seminal contributions to the field of  electronic structure theory have been an inspiration to all of us over the years. The authors would like to thank the anonymous reviewers whose comments and suggestions have greatly improved this manuscript. We also thank the anonymous referee for providing a useful writeup about the EIP-reformulation of FSCC method. We acknowledge Dr. Sangita Sen for her assistance.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Financial support by the CSIR of India [Ref. no. 01(2973)/19/EMR-II] is gratefully acknowledged by S. Chattopadhyay (grant no. 01(7836)/19).