THE ISOTOPE EFFECT IN ELECTRONIC EXPECTATION VALUES - AN ALL-QUANTUM STUDY OF C6H6 AND C6D6

A Feynman path integral Monte Carlo (PIMC) formalism and an electronic ab initio Hamiltonian have been linked together to study C6H6 and C6D 6 molecules with consideration of the quantum character of the atomic nuclei and the electrons. Electronic expectation values of the two sys tems have been evaluated as averages over manifolds of nuclear configu rations accessible in thermal equilibrium (canonical ensemble statisti cs). These averages have been correlated with expectation values of th e electronic Hamiltonian derived for the optimized geometry. The theor etical set-up employed leads to a temperature and isotope dependence o f pure electronic quantities. On the basis of these all-quantum result s we discuss the justification of extrapolating from the observed isot ope effect in the superconducting transition temperature to yield an e lectron-phonon-based pairing mechanism.