Theoretical study of the He–HF+ complex. I. The two asymptotically degenerate ground state potential energy surfaces

Abstract

Two three-dimensional potential energy surfaces ͑PESs͒ are reported for the cationic complex He-HF ϩ ; they are degenerate for linear geometries of the complex and correlate with the doubly degenerate X 2 ⌸ ground state of the HF ϩ monomer. The PESs are computed from the interaction energies of the neutral dimer and the ionization potentials of the He-HF complex and the HF molecule. Ionization potentials are obtained from the outer valence Green's function ͑OVGF͒ method, while the energies of the neutral species are computed by means of the single and double coupled-cluster method with perturbative triples ͓CCSD͑T͔͒. For comparison, interaction energies of the ionic complex were computed also by the use of the partially spin-restricted variant of the CCSD͑T͒ method. After asymptotic scaling of the OVGF results, good agreement is found between the two methods. A single global minimum is found in the PES, for the linear He-HF ϩ geometry. The well depth and equilibrium separation are 2.240 Å and 1631.3 cm Ϫ1 , respectively, at an HF ϩ bond length rϭ1.0012 Å, in rather good agreement with results of Schmelz and Rosmus ͓Chem. Phys. Lett. 220, 117 ͑1994͔͒. The well depth depends much more strongly on the internuclear H-F separation than in the neutral He-HF complex and the global minimum in a full three-dimensional PES occurs at rϭ1.0273 Å.

Jahn–Teller effect in van der Waals complexes; Ar–C6H6+ and Ar–C6D6+

“…9,10 Also, unscaled versions of the IP method have been applied, [33][34][35] however. As was shown 10 on the example of He-HF ϩ , the method can also be applied to open-shell monomers with degenerate electronic states, HF ϩ (X 2 ⌸) in this case, to obtain multiple asymptotically degenerate potential surfaces. The two potentials in this example correspond to electronic states of the complex with AЈ and AЉ symmetry.…”

“…It is therefore appropriate to scale only C 4,0 . In previous work 9, 10 we scaled also C 5,1 but this coefficient is zero in the present case. The polarizability of Ar obtained from the fitted coefficient C 4,0 , 8.63 a 0 3 , is significantly lower than the accurate value of 11.08 a 0 3 and C 4,0 was therefore scaled by the ratio 11.08/8.63.…”

“…Binding in cationic complexes is typically an order of magnitude stronger than in the corresponding neutral systems. 9,10 Experiments revealed, however, that the binding of the ionic complex in this case is not much stronger. The binding energies D e and D 0 are 387 and 328 cm Ϫ1 in neutral argonbenzene according to the ab initio calculations 6 and slightly less according to experiment, 11 while it is known from ionization energies 12 that D 0 is only 170 cm Ϫ1 larger in the ionic complex.…”

“…For the ab initio calculation of the adiabatic potential energy surfaces we applied a special method for cationic complexes 9,10 that is both convenient and efficient. In Sec.…”

Theoretical study of the He–HF+ complex. II. Rovibronic states from coupled diabatic potential energy surfaces

“…The preceding paper, 1 from now on referred to as Paper I, presents the calculation of the two asymptotically degenerate adiabatic potential surfaces of the He-HF ϩ complex that correlate with the degenerate X 2 ⌸ ground state of HF ϩ . The twofold spatial degeneracy of this ⌸ state is lifted, except when the complex has a linear geometry.…”