### The Journal of Chemical Physics -- September 15, 1996 -- Volume
105, Issue 11, pp. 4751-4760

### Representing molecular shape and interactions: A reduced
intermolecular potential for copper phthalocyanine

Da-Jiang Liu, Robin L. Blumberg Selinger, John D. Weeks
### Abstract:

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.