We developed potential functions for (HCN)(2) and used them to study larger aggregates. The functions are based on ab initio perturbation calculations for the dimer and on molecular properties also estimated from ab initio calculations involving two different basis sets. The overall energy is resolved into electrostatic, repulsion, induction, and dispersion components. The electrostatic contribution is represented by a multipole expansion at several sites. The repulsion function is of the exponential type and includes atomic anisotropy. The dispersion is described by means of a London-type formula. Finally, the induction contribution is expressed in terms on polarizabilities distributed among the atoms. The proposed functions accurately reproduce the characteristics of the HCN dimer and the two minimum-energy forms of the trimer. Also, they predict various properties (e.g., the dependence-of the dipole moment and interaction energy on cluster size) of larger aggregates.