The potential energy surface of the ground state He+Cl2(1Sgr;g) is calculated by using the perturbation theory of intermolecular forces and supermolecular Moslash;llerndash;Plesset perturbation theory approach. The potential energy surface of the first excited triplet He+Cl2(3Pgr;u) was evaluated using the supermolecular unrestricted Moslash;llerndash;Plesset perturbation theory approach. In the ground state two stable isomers are found which correspond to the linear Hendash;Clndash;Cl structure (a primary minimum,De=45.1 cmminus;1,Re=4.25 Aring;) and to the Thyphen;shaped structure with He perpendicular to the molecular axis (a secondary minimum,De=40.8 cmminus;1,Re=3.5 Aring;). The small difference between these geometries is mainly due to the induction effect which is larger for the linear form. The results obtained for the Thyphen;shaped minimum are in good agreement with the excitation spectroscopy experiments which observed only the Thyphen;shaped form lsqb;Beneventietal., J. Chem. Phys.98, 178 (1993)rsqb;. In the lowest triplet states correlating with Cl2(3Pgr;u),3Arsquo;and3Alsquo;, the same two isomers correspond to minima. Now, however, the Thyphen;shaped form is lower in energy. The3Arsquo;and3Alsquo;states correspond to (De,Re) of (19.9 cmminus;1, 3.75 Aring;) and (30.3 cmminus;1, 3.50 Aring;), respectively, whereas the linear form is characterized by (19.8 cmminus;1, 5.0 Aring;). The binding energy for the T form in the lower3Alsquo;state is in good agreement with the experimental value of Beneventietal.
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