Starting from an accurate inter-atomic potential we develop a simple scheme of generating an "on-lattice" molecular potential of short range, which is then incorporated into a lattice Monte Carlo code for simulating size and shape evolution of nanocrystallites. As a specific example, we test such a procedure on the morphological evolution of a molecular crystal of interest to us, e.g., Pentaerythritol Tetranitrate, or PETN, and obtain realistic facetted structures in excellent agreement with experimental morphologies. We investigate several interesting effects including, the evolution of the initial shape of a "seed" to equilibrium configuration, and the variation of growth morphology as a function of the rate of particle addition relative to diffusion. Crown Copyright (c) 2006 Published by Elsevier B.V. All rights reserved.