Isotope dependence of dispersion forces: a Feynman path integral-ab initiostudy

The Feynman path integral Monte Carlo (PIMC) technique has been combined wi th an uh initio Hartree-Fock Hamiltonian in order to study the influence of isotope effects on dispersion interactions between non-polar (hydrocarbon) molecules. The PIMC formalism renders possible the consideration of the nu clear degrees of freedom in the evaluation of electronic expectation values : it corresponds to an adiabatic Born-Oppenheimer description of molecules. The nuclear fluctuation lead to the formation of temporary dipole moments \mu\ in thermal equilibrium which are employed as key parameter for on anal ysis of isotope effects in van der Waals bonding. Different benzene and eth ylene isomers have been considered as model compounds in the present PIMC-a b initio simulations. The theoretical data derived are correlated with avai lable experimental observations. (C) 2001 Elsevier Science B.V. All rights reserved.