New Ab Initio Potential Energy Surfaces for NH₃ Constructed from Explicitly Correlated Coupled-Cluster Methods

Abstract

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New Theoretical Infrared Line List for the Methyl Radical with Accurate Vibrational Band Origins from High-Level Ab Initio Calculations

“…This analytical representation, which converges quite rapidly despite the out-ofplane bending vibration, has been recently used to describe the NH 3 PES with its two minima separated by a barrier of 2000 cm −1 . 35 The D 3h symmetry coordinates associated with the symmetric and asymmetric stretching and in-plane bending vibrations are defined using the projection technique 88 as follows (2 )/ 6…”

“…Because of the high-quality but costly basis sets and corrections beyond the RHF-UCCSD(T) level employed in this work to construct the PES, two different grids of nuclear configurations have been considered. A similar strategy was already applied for other molecules (see, e.g., CH 4 , 99 H 2 CO, 100 and NH 3 35 ). The first grid (called grid_1) was generated by varying simultaneously all the six coordinates defined using eqs 1 and 6.…”

“…In this work, the CH 3 PES was constructed by following the same approach as for NH 3 in ref 35. Both even and odd powers were allowed for the S 2 symmetry coordinate, which transforms as the totally symmetric representation (see eq 4).…”

“…The PES and DMS of CH 3 are both expressed in this work as a Taylor power series expansion in terms of symmetry coordinates S i . This analytical representation, which converges quite rapidly despite the out-of-plane bending vibration, has been recently used to describe the NH 3 PES with its two minima separated by a barrier of 2000 cm –1 . The D 3 h symmetry coordinates associated with the symmetric and asymmetric stretching and in-plane bending vibrations are defined using the projection technique as follows where y k and φ k with k = 1, 2, and 3 are functions depending on the internal coordinates.…”

“…The ab initio calculations reported in this work were performed by using the MOLPRO, , MRCC, , and CFOUR , packages while the nuclear motion calculations were carried out using our homemade variational computer code TENSOR − applied with success in refs – for computing accurate spectroscopic line lists of semirigid molecules. As in ref , a hybrid nonrigid Hamiltonian model was built in this work using the Hougen–Bunker–Johns formalism, where the relation between the curvilinear ρ̅ and effective coordinates ρ was explicitly established in ref .…”