Two algorithms for defining atoms and molecules in condensed phases

Two algorithms for defining single entities in condensed phase based on the calculation of a zero flux surface of the gradient of the electron density are proposed. First the two approaches are applied to a water molecule in a liquid system; the different behaviour of the two algorithms is graphical ly shown in the planar section of the molecular plane (i.e., HOH plane of t he molecule analyzed). Next, using the two algorithms for partitioning elec tron density, the average molecular dipole moment of the water molecule in liquid phase is calculated averaging over several configurations; analogies and differences with other methods are reported and their physical interpr etation is discussed. There exist different approaches for calculating the zero flux surface (or equivalently the volume defined by such a surface); t he two described in this work differ from the others because the calculatio n in both cases is straightforward (i.e., the surface is determined directl y from the data corresponding to the electron density) and is characterized by a relatively short computational time.