Ground state structure of C5H5 and van der Waals interaction with He and Ne

A geometry optimization of the two symmetry forms, D-5h and C-2v, Of C5H5 w as performed at the second-order perturbation theory, MP2, using the 6-311+G** basis set. Using these two optimized geometries of D-5h and C-2v symme tries, single point calculations were performed including high-order electr on correlation effects. At the highest level considered, CCSD (T), the (2)A (2) State of C-2v symmetry lies below the E-2(1)n state of D-5h symmetry by 0.20 eV. Next we analyze the interaction energy between C5H5 and He and Ne with the C5H5 monomer held fixed at its optimized (2)A(2) geometry. The po tential energy curve is obtained using single point MP2 calculations with H e and Ne placed along the center of the C5H5 ring. Whereas for He the calcu lations indicate that the complex is probably not stable, in the case of Ne the complex is certainly bound. The best estimate of the binding energy gi ves a value of similar to 100 cm(-)1. (C) 1999 Elsevier Science B.V. All ri ghts reserved.