Analysis of orientational autocorrelation in model glass forming system using MD simulation

The super-Arrhenian temperature dependence of mobility in glass forming materials is but one of several long-standing problems in condensed matter physics. Equally as intriguing is the origin of the α- and β-relaxation processes and the broadness of the relaxation spectrum. It is hoped that the molecular simulations can provide significant insight and yet there are surprisingly few studies of the relaxation spectrum as compared to the temperature dependence. We perform an MD study of a model system of molecular dumbbells, where the relaxation spectrum is extracted from the decay of the orientational autocorrelation functions P1 and P2, that is the Legendre polynomials of the cosine of the angle of rotation of molecular axis. The autocorrelation functions are analyzed with the Prony series, where both α- and β-relaxation processes are well resolved. The α-process is single-exponential at high temperature but broadens as the temperature decreases. We identify the molecular level events that accompany the broadening of the spectrum. The implications of the emerging picture of the molecular motions for the theory of the glass transition will be discussed.