The isotope effect in electronic expectation values: an all-quantum study of C6H6 and C6D6

Abstract

A Feynman path integral Monte Carlo (PIMC) formalism and an electronic ab initio Hamiltonian have been linked together to study C6H6 and C6D6 molecules with consideration of the quantum character of the atomic nuclei and the electrons. Electronic expectation values of the two systems have been evaluated as averages over manifolds of nuclear configurations accessible in thermal equilibrium (canonical ensemble statistics). These averages have been correlated with expectation values of the electronic Hamiltonian derived for the optimized geometry. The theoretical set-up employed leads to a temperature and isotope dependence of pure electronic quantities. On the basis of these all-quantum results we discuss the justification of extrapolating from the observed isotope effect in the superconducting transition temperature to yield an electron-phonon-based pairing mechanism.