Static-lattice theory of crystalline HCN. I. Effective dipole potential

An intermolecular potential is constructed for HCN in its two crystalline modifications. The long-range part is treated as the electrostatic interaction of point effective dipoles obtained using dielectric data, lattice summations and molecular-orbital calculation. The short-range part is represented by a symmetrical dumb-bell model with 6:12 potentials, a simple form chosen to clarify the role of the effective dipoles. It is parametrized to yield the correct static-lattice energy at the observed tetragonal structure while being consistent with gas-phase second virial coefficients. A generally satisfactory understanding of both crystal structures is obtained, although no stable orthorhombic structure is found. The potential is readily used for calculating piezoelectric properties, as in the following paper.