An effective Hamiltonian for correlations in interfaces

The effective Hamiltonian H-cap for a liquid-vapor interface is constructed by using the "experimental" direct correlation function obtained from mole cular-dynamics simulation. The density fluctuation is parametrized accordin g to: (a) The standard shift of the density profile, parallel and small; (b ) the parallel shift; and (c) the shift with rotation as recently introduce d, used in a density functional theory and compared with experiment. We fin d a universal shape of H-cap, discussed in some detail. The shape strongly suggests the existence of two regions of low wave-vector q and of high q co rresponding to microscopic distances. Although bending and nonzero curvatur e of the fluctuating interface are present, the apparent bending (rigidity) coefficient is always negative, thus supporting the contention that the me mbrane Hamiltonians are not applicable to normal liquid-vapor interfaces. S everal definitions of the momentum-dependent surface tension are also discu ssed. (C) 2001 American Institute of Physics.