A NEW ab initio POTENTIAL ENERGY SURFACES CALCULATION OF THE CH(X)+He () VAN DER WAALS COMPLEX D. Ben Abdallah*, N. Jaidane, Z. Ben Lakhdar Laboratoire de Physique Atomique et Moléculaire, Département de Physique, Faculté des Sciences, Université de Tunis-Elmanar. Le Belvédère, 1060 Tunis, Tunisie * Corresponding author. E-mail : Driss.BenAbdallah@fst.rnu.tn Received : 24 May 2002; revised version accepted : 18 February 2003
Abstract The potential energy surfaces of CH + He, which correlates asymptotically with CH (X ) + He (), have been calculated using the coupled electron pair approximation (CEPA) and large basis sets. The asymptotically degenerate and states split respectively into two states of and symmetry when the symmetry of the diatomic molecule is lifted by the approach of the noble gas atom. It has been found that the two states are distinctly different. Although qualitatively very similar to our potential energy surfaces (PES’s) ( Wagner et al [18] and Cybulski et al [19] ), the new PES’s have substantially deeper wells (De =60 cm^{-1}). The PES’s of the state has only a single and relatively deep minimum De 60cm^{-1} for the T–shaped geometry, at R = 5.2 a.u and = 80°. The minimum is unusually deep for a complex of He, and it can be viewed as an example of an incipient chemical bond. In contrast, the state’s PES represents a typical van der Waals interaction, which could be characterized by two similarly deep minima. The first minimum occurs for the collinear He – C – H arrangement, at R 7.5 a.u and = 180°, and is 10 cm^{-1} deep. The second minimum has a troughlike form which joins the region between R 7.5 a.u, = 0° and R 8.0 a.u, = 40°. The average and half difference of the calculated points on the, and potential energy surfaces were fitted to analytical functions (Legendre Polynomial expansion). Keywords: ab-intio; Potential; Molecules-collision. |
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