Ab initio studies of linear and nonlinear optical polarizabilities of molec
ular urea clusters are reported with on eye toward accounting for the valid
ity of oriented-gas model of molecular crystals. The first-order hyperpolar
izabilities of the nearest-neighbor structure of urea cluster are reported
for the first time. We further propose a shell-structure model to improve t
he oriented-gas model for reliably calculating nonlinear optical susceptibi
lity molecular crystals.