Global analytical potential energy surface for the electronic ground state of NH3 from high level ab initio calculations

Roberto Marquardt, Kenneth Sagui, Jingjing Zheng, Walter Thiel, David Luckhaus, Sergey Yurchenko, Fabio Mariotti, Martin Quack

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Abstract

The analytical, full-dimensional, and global representation of the potential energy surface of NH3 in the lowest adiabatic electronic state developed previously (Marquardt, R.; et al. J. Phys. Chem. B 2005, 109, 8439-8451) is improved by adjustment of parameters to an enlarged set of electronic energies from ab initio calculations using the coupled cluster method with single and double substitutions and a perturbative treatment of connected triple excitations (CCSD(T)) and the method of multireference configuration interaction (MRCI). CCSD(T) data were obtained from an extrapolation of aug-cc-pVXZ results to the basis set limit (CBS), as described in a previous work (Yurchenko, S.N.; et al. J. Chem. Phys 2005, 123, 134308); they cover the region around the NH3 equilibrium structures up to 20 000 hc cm -1. MRCI energies were computed using the aug-cc-pVQZ basis to describe both low lying singlet dissociation channels. Adjustment was performed simultaneously to energies obtained from the different ab initio methods using a merging strategy that includes 10 000 geometries at the CCSD(T) level and 500 geometries at the MRCI level. Characteristic features of this improved representation are NH3 equilibrium geometry req(NH 3) ≈ 101.28 pm, αeq(NH3) ≈ 107.03, the inversion barrier at rinv(NH3) ≈ 99.88 pm and 1774 hc cm-1 above the NH3 minimum, and dissociation channel energies 41 051 hc cm-1 (for NH3 → ( 2B2)NH2 + (2S1/2)H) and 38 450 hc cm-1 (for NH3 → (3Σ -)NH +(1Σg+)H2); the average agreement between calculated and experimental vibrational line positions is 11 cm-1 for 14N1H3 in the spectral region up to 5000 cm-1. A survey of our current knowledge on the vibrational spectroscopy of ammonia and its isotopomers is also given.

Original languageEnglish (US)
Pages (from-to)7502-7522
Number of pages21
JournalJournal of Physical Chemistry A
Volume117
Issue number32
DOIs
StatePublished - Aug 15 2013

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