ELECTRONS AND NUCLEI OF C6H6 AND C6D6 - A COMBINED FEYNMAN PATH-INTEGRAL - AB-INITIO APPROACH

We have linked an ab initio approach of the Hartree-Fock (HF) type to the Feynman path integral quantum Monte Carlo (PIMC) formalism in orde r to study C6H6 and C6D6 under consideration of the quantum character of the nuclei and electrons. The combination of the statistical Monte Carlo approach with an electronic Hamiltonian offers the possibility t o study the influence of the quantum and classical (= thermal) nuclear degrees of freedom on electronic expectation values. The PIMC techniq ue has been used to derive the finite-temperature properties of the nu clei of both pi rings. We discuss the temperature (T) dependence of th e energy of the C6H6 and C6D6 nuclei, their spatial delocalization pro perties as well as the radial and angular distribution functions. The nuclear configurations generated by the PIMC formalism have been used as input for ab initio HF calculations. Electronic expectation values have been derived as ensemble averages over 6000 different nuclear con figurations which are populated in thermal equilibrium. As a result of the large quantum effects of the C6H6 and C6D6 nuclei, we derive ense mble averaged electronic expectation values which differ sizeable from the corresponding single-configuration quantities at the energy minim um. This difference is mainly caused by nuclear quantum effects; therm al degrees of freedom are of minor importance only. The electronic ori gin of the potential energy part of the total vibrational energy is em phasized. It is largely determined by the raise in the electron-core a ttraction under the influence of the spatial uncertainty of the nuclei . The all-quantum approach yields a temperature and isotope dependence of bare electronic quantities already in the framework of the Born-Op penheimer approximation (BOA). The principal findings of the all-quant um study have been used to reconsider certain solid state problems. We mention theoretical difficulties to reproduce Compton profiles and co nsider metal-insulator transitions of the Mott type as well as superco nductivity. On the basis of the present all-quantum results we have to emphasize, that there is no unambiguous justification to adopt an obs erved isotope shift in the superconducting transition temperature T-c as an indicator of an electron-phonon-coupled superconducting pairing mechanism. (C) 1998 Elsevier Science B.V.