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We examine the distinct part of the density autocorrelation function Fd(q,t),also called the intermediate scattering function, from the point of view of thevibration-transit (V-T) theory of monatomic liquid dynamics. A similar studyhas been reported for the self part, and we study the self and distinct partsseparately because their damping processes are not simply related. We beginwith the perfect vibrational system, which provides precise definitions of theliquid correlations, and provides the vibrational approximation Fdvib(q,t) atall q and t. Two independent liquid correlations are defined, motional andstructural, and these are decorrelated sequentially, with a crossover timetc(q). This is done by two independent decorrelation processes: the first,vibrational dephasing, is naturally present in Fdvib(q,t) and operates to dampthe motional correlation; the second, transit-induced decorrelation, is invokedto enhance the damping of motional correlation, and then to damp the structuralcorrelation. A microscopic model is made for the "transit drift", the averagedtransit motion that damps motional correlation on 0 < t < tc(q). Following thepreviously developed self-decorrelation theory, a microscopic model is alsomade for the "transit random walk," which damps the structural correlation on t> tc(q). The complete model incorporates a property common to both self anddistinct decorrelation: simple exponential decay following a delay period,where the delay is tc(q, the time required for the random walk to emerge fromthe drift. Our final result is an accurate expression for Fd(q,t) for all qthrough the first peak in Sd(q). The theory is calibrated and tested usingmolecular dynamics (MD) calculations for liquid Na at 395K; however, the theoryitself does not depend on MD, and we consider other means for calibrating it.

Application of vibration-transit theory to distinct dynamic response for a monatomic liquid