  We propose a very simple but realistic enough model which allows to include a
large number of molecules in molecular dynamics MD simulations of these
bilayers, but nevertheless taking into account molecular charge distributions,
flexible amphiphilic molecules and a reliable model of water. All these
parameters are essential in a nanoscopic scale study of intermolecular and long
range electrostatic interactions. This model was previously used by us to
simulate a Newton black film and in this paper we extend our investigation to
bilayers of the biological membrane type. The electrostatic interactions are
calculated using Ewald sums and, for the macroscopic long range electrostatic
interactions, we use our previously proposed coarsed fit of the (perpendicular
to the bilayer plane) molecular charge distributions with gaussian
distributions. To study an unique biological membrane (not an stack of
bilayers), we propose a simple effective external potential that takes into
account the microscopic pair distribution functions of water and is used to
simulate the interaction with the surrounding water. The method of effective
macroscopic and external potentials is extremely simple to implement in
numerical simulations, and the spatial and temporal charge inhomogeneities are
then roughly taken into account. Molecular dynamics simulations of several
models of a single biological membrane, of neutral or charged polar
amphiphilics, with or without water (using the TIP5P intermolecular potential
for water) are included.
