ON THE CORRESPONDENCE BETWEEN CLASSICAL AND QUANTUM-MECHANICS IN MACROMOLECULAR SYSTEMS - TOO MUCH CLASSICAL CHAOS

In a recent study we found the classical dynamics of a polyethylene (P E) chain to exhibit low dimensional chaos at temperatures as low as a few Kelvin. These results strongly suggest that classical molecular dy namic simulations in polymer systems can grossly overestimate vibratio nal motion, which consequently results in disordered structures. In co ntrast, quantum mechanical calculations using Internal Coordinate Quan tum Monte Carlo (an improved method with an initial conjecture for the correct wave function) indicate that the quantum ground state for a t hree-dimensional model PE chain is far more rigid than determined from molecular dynamics (MD) simulations, even at energies as low as a sma ll fraction of the ground state energy This result casts uncertainty o n the reliability of MD estimates of dynamical or structural quanti ti es relevant to the study of some macromolecular systems.