The Composed Approximation for finite difference modeling of a solid free s
urface has previously been found to have serious stability problems for sig
nificant ranges of the physical parameters. Here it is shown that these pro
blems can always be avoided by choosing the grid spacings to be unequal in
the two directions, with appropriate ratios of the two spacings. The demons
tration involves an explicit construction of the main unstable mode, which
shows its dependence on the grid-space ratio. This analysis is backed up by
numerical tests that show that given an appropriate choice of the spacing
ratio-as deduced from the explicit construction-there are no serious stabil
ity problems over a full range of the physical parameters. The Composed App
roximation in this format gives accurate results for a wide class of physic
al situations.