High excitations in coupled-cluster series: Vibrational energy levels of ammonia

Timo Rajamäkl, Mihály Kállay, Jozef Noga, Pierre Valiron, Lauri Halonen

Research output: Contribution to journalArticle

53 Citations (Scopus)


The ammonia molecule containing large amplitude inversion motion is a revealing system in examining high-order correlation effects on potential energy surfaces. Correlation contributions to the equilibrium and saddle point geometries, inversion barrier height and vibrational energy levels, including inversion splittings, have been investigated. A sixdimensional Taylor-type series expansion of the Born-Oppenheimer potential energy surface, which is scaled to different levels of theory, is used to determine vibrational energy levels and inversion splittings variationally. The electronic energies are calculated by coupled-cluster methods, combining explicitly correlated R12 theory (which includes the interelectronic coordinate in the electronic wave function) with a conventional approach including excitations up to the pentuple level. Finally, the electronic correlation contribution is scaled to the full configuration interaction limit. Corrections due to relativistic and non-Born-Oppenheimer effects are also included. Special emphasis is put on the convergence of the high-order contributions with respect to the size of the atomic basis set. To achieve an accuracy of 1 cm-1, it is essential to be at the basis set limit, include all the subtle effects and also include highly excited configurations-even up to the pentuple level in the coupled-cluster expansion.

Original languageEnglish
Pages (from-to)2297-2310
Number of pages14
JournalMolecular Physics
Issue number21-22
Publication statusPublished - Nov 10 2004

ASJC Scopus subject areas

  • Biophysics
  • Molecular Biology
  • Condensed Matter Physics
  • Physical and Theoretical Chemistry

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