Low-lying valence excited states of CNC, C₂N, N₃, and NCO studied using the electron-attached and ionized symmetry-adapted-cluster configuration-interaction and equation-of-motion coupled-cluster methodologies

Abstract

Low-lying valence excited states of four open-shell triatomic molecules, CNC, C₂N, N₃, and NCO, are investigated using the electron-attached (EA) and ionized (IP) symmetry-adapted-cluster configuration-interaction (SAC-CI) general-R as well as the full and active-space EA and IP equation-of-motion coupled-cluster (EOMCC) methods. A comparison is made with experiment and with the results of the completely renormalized (CR) CC calculations with singles, doubles, and non-iterative triples defining the CR-CC(2,3) approach. Adiabatic excitation energies of the calculated states are in reasonable agreement with the experimental values, provided that the 3-particle–2-hole (3p-2h) components in the electron attaching operator, as in the EA SAC-CI SDT-R and EA EOMCCSD(3p-2h) approaches, are included in the calculations for the excited states of C₂N and CNC which have a predominantly two-electron character. The results also reveal that the active-space EA/IP EOMCC schemes with up to 3p-2h/3h-2p excitations are able to accurately reproduce the results of their much more expensive parent methods while requiring significantly less computational effort. Furthermore, the more ‘black-box’ CR-CC(2,3) approach calculates the lowest state of each symmetry with the same accuracy as that obtained with the EA/IP SAC-CI SDT-R and EA/IP EOMCCSD(3p-2h/3h-2p) methods, confirming the significance of higher-order correlation effects in obtaining an accurate description of excited states of radicals, particularly the valence excited states of the CNC and C₂N species dominated by two-electron processes.

Keywords:

• valence excited states
• open-shell molecules
• symmetry-adapted-cluster configuration-interaction method
• equation-of-motion coupled-cluster method
• ab initio calculations

Acknowledgement

We dedicate this paper to Professor Henry F. Schaefer III in celebration of his 65th birthday. One of us (P.P.) would like to thank Professors T. Daniel Crawford and C. David Sherrill for inviting him to write a contribution for a special issue of Molecular Physics in honour of Professor Henry F. Schaefer III. We would also like to thank Professor Hiroshi Nakatsuji for useful discussions and interest in this work. This study was supported by JST, CREST and a Grant-in-Aid for Scientific Research in Priority Areas ‘Molecular Theory for Real Systems’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan (M.E.), the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy [Grant No. DE-FG02-01ER15228] (P.P), and the National Science Foundation Graduate Research Fellowship (J.R.G.). In addition, some of the calculations were performed on the computer systems provided by the High Performance Computing Center at Michigan State University.