Structure of Liquids. IV. Direct Correlation Functions of Liquid Argon

Abstract

Recently acquired x‐ray diffraction data from 13 states of fluid argon were used to compute the direct correlation function, C(r). This function is found to be definitely short range, particularly when compared to the conventional radial distribution function at the same state. Using experimental data and knowledge of the radial distribution function, estimates were also made of the state‐dependent behavior of C(r) for radii less than the atomic radius. The median location of the first zero of C(r) is r₀=3.39±0.06 Å; this value tends to increase with increasing temperature and with decreasing density. The median location of the maximum in C(r) is r_max=3.84±0.07 Å; this value shows a slight but inconclusive tendency to increase with increasing temperature and to decrease with increasing density. The height of the single maximum varies from 0.8 to 1.3 over the range studied; while there was no observable temperature effect, the peak height tends to decrease as the density is increased. The computed direct correlation functions were used in conjunction with previously calculated radial distribution functions in order to test fundamental hypotheses underlying the derivations of the Percus—Yevick and convoluted‐hypernetted‐chain integral equations, with the conclusion that these equations have systematic density‐dependent defects. The intermolecular potential function was estimated by zero density extrapolation of a potential computed from the PY equation, with results in good agreement with currently accepted values for the pair potential.