The excited, ionized and electron attached states within the EOM-CC approach with full inclusion of connected triple excitations

Abstract

A full inclusion of the connected single (S), double (D) and triple (T) excitations into the equation-of-motion (EOM) coupled-cluster (CC) approach is discussed in the context of the calculations of excitation energies (EE), ionization potentials (IP) and electron affinity (EA). The EOM-CC formalism relies on the diagonalization of the CI-like matrix representing similarity transformed Hamiltonian H , defined as where T is a cluster operator. The H operator is non-Hermitian and – in the case of the EOM-CCSDT model – includes also three- and four-body elements which have to be treated in a special factorized manner to avoid very high scaling of the method and – at the same time – to keep the method fully rigorous. An analysis of the results indicates that the inclusion of triples for the EE reduces the average error twice and for the IP – three times. The performance of the EOM-CCSDT scheme for the electron affinity is more difficult to assess due to scarcity of the experimental data.

Keywords:

• equation-of-motion CC
• connected triple excitations
• excitation energies
• ionization potentials
• electron affinity

Acknowledgements

It is a great pleasure to contribute this paper to a special volume celebrating 50th Sanibel Symposium and work done in Quantum Theory Project in Gainesville. I would like to express my gratitude for the hospitality of Professor Rodney J. Bartlett, faculty members and staff during my stay in QTP.

Current position: Assistant Professor, Institute of Chemistry, University of Silesia, Katowice, Poland.

Quantum Theory Project: Postdoctoral Associate (2002–2003); several 2–3 months visits in the years 2004–2010.

This work has been supported by the Ministry of Science and Higher Education, Poland, under grant no. N N204 090938.