Refined ab initio intermolecular ground-state potential energy surface for the He–C₂H₂ van der Waals complex

Abstract

A refined CCSD(T) intermolecular potential energy surface is developed for the He–C₂H₂ van der Waals complex. For this, 206 points on the intermolecular potential energy surface, evaluated using the CCSD(T) method and the aug-cc-pVQZ basis set extended with a set of 3s3p2d1f1g midbond functions, are fitted to a 15-parameter analytic function. The potential is characterised by minima of −24.21 cm⁻¹ at distances between the rare gas atom and the C₂H₂ centre of mass of 4.3453 Å, and with the complex in a linear configuration. At intermediate distances the surface is rather similar to that developed previously by Munteanu and Fernández (J. Chem. Phys., 123, 014309, 2005) but differs notably at short range. The improved potential energy surface should, therefore, be particularly useful for computations of collision line broadening. Dynamical calculations of a number of rovibrational bound state energies and wave functions are presented. Inspection of the nodal surfaces of several low lying excited states shows that the complex is close to the free rotor limit.

Keywords:

• He–C₂H₂ van der Waals complex
• intermolecular potential
• rovibrational energies

Acknowledgements

It is a pleasure to dedicate this paper to Trygve Helgaker, a friend and a colleague who turns sixty this year. We thank Professor Ad van der Avoird for supplying us with a FORTRAN routine to compute the He–C₂H₂ potential energy of Ref. [7]. This work has been supported by the U.S. National Science Foundation through grant CHE-0718547 and by the Ministerio de Ciencia e Innovación (CTQ2011-29311-C02-01 project). We thank the Centro de Supercomputación de Galicia (CESGA) for computing time.