The potential energy between molecules is often represented as a sum of pairwise additive potentials for all atom pairs in both molecules. Such atomistic potentials encode much physical and chemical information, and in particular give an accurate representation of the molecular shape. However, the number of terms in the sum for a pair of molecules goes as N-2 where N is the number of atoms in a molecule, and thus grows rapidly inefficient for large N. Starting with an atomistic pairwise additive potential for Copper Phthalocyanine (CuPc), a planar tile-shaped molecule with 57 atoms, we determine a simpler reduced intermolecular potential consisting of a sum of effective pair interactions between 13 appropriately chosen "interaction sites" on each molecule. This potential reproduces many qualitative features of the full atomistic potential model for CuPc including the very anisotropic molecular shape, but is much easier to evaluate numerically, requiring only 1% as much computation time as the full atomistic potential. Crystal structures of CuPc using both the atomistic and reduced potentials are determined and compared, and a discussion of diffusion barriers is given. Some of the general issues and physical considerations that arise when attempting this reduction are discussed along with other possible applications of these ideas.