AN INTERNAL COORDINATE QUANTUM MONTE-CARLO METHOD FOR CALCULATING VIBRATIONAL GROUND-STATE ENERGIES AND WAVE-FUNCTIONS OF LARGE MOLECULES -A QUANTUM GEOMETRIC STATEMENT FUNCTION-APPROACH

An internal coordinate quantum Monte Carlo (ICQMC) method using import ance sampling is illustrated for a 100 atom model polyethylene chain. Importance sampling with an internal coordinate guiding wave function yields smoother, more physically reasonable wave functions and lower g round state energies than Cartesian importance sampling, in good agree ment with normal coordinate analysis results. A novel geometric statem ent function (GSF) method for economizing expressions involving first and second derivatives of stretch, bend, and torsion internal coordina tes by up to 2 orders of magnitude allows QMC calculations to be perfo rmed even for large molecules in reasonable times on standard workstat ions. The ICQMC method with quantum GSF is eminently suitable for larg e molecules with complicated, strongly coupled potential energy surfac es such as polymer chains.