Comparative study of Møller–Plesset perturbation theory and configuration interaction theory for the structure of the N4+ cation

✍ Scribed by Gary Kemister; J. Barrie Peel

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: English
• Weight: 480 KB
• Volume: 28
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/oms.1210280408

✦ Synopsis

Abstract

A theoretical study of the ground electronic state of N~4~^+^, the dimer ion of N~2~, has shown, in agreement with other studies, that the Møller–Plesset perturbation expansion for open‐shell molecular species is unstable and divergent when used with certain spin‐contaminated UHF reference wavefunctions. This failure is more serious than in the earlier studies, which noted that the breakdown occurred for the extended bond lengths descriptive of potential energy curves of diatomic systems. In this study the breakdown occurs for the potential energy surface of N~4~^+^ close to the global minimum structure, which is incorrectly determined as corresponding to a trans‐planar geometry. In comparison variational configuration interaction procedures represented by the CISD and QCISD methods are clearly superior in the case of N~4~^+^, with both yielding the correct linear geometry. The QCISD calculation provides a calculated antisymmetric vibration frequency which is in good agreement with, and hence confirms, the assignment of the neon‐matrix infra‐red spectrum attributed to the N~4~^+^ species. The search for a bound excited electronic state of N~4~^+^ was partly successful in that a configuration of rectangular geometry shows properties in fair agreement with experimental results.