Two-body correlations in two-dimensional classical solids and their role in first-order perturbation theories

A highly accurate theoretical approach, based on exact and quasiexact sum r ules, is presented for the averaged two-body correlation function of the ha rd-disk solid. Excellent results are obtained as compared to computer simul ations for this function. However, when these results are used as the basis for a first-order perturbation theory, the phase diagram of the two-dimens ional square-well model potential is only reproduced qualitatively, showing that higher-order correlation terms play a non-negligible role in two dime nsions. (C) 2000 American Institute of Physics. [S0021-9606(00)50920-9].