WEAK-INTERACTIONS IN EXTENDED HCN CHAINS

Model calculations have been carried out on extended linear chains of up to 50 HCN molecules in order to assess the effects of pairwise vers us non-pairwise intermolecular interactions. The contraction in separa tion distance from the dimer to the infinite chain crystalline structu re is known to be 0.08-0.10 Angstrom. Pairwise interactions lead to a share of this contraction as non-nearest neighbor interactions take ef fect. Direct polarization, a three-body interaction, adds the bulk of the remaining contraction, and full treatment of back polarization yie lds a net contraction of 0.076 Angstrom. Three-body multipole dispersi on will also contribute to the contraction. The per monomer dipole mom ent grows by 0.8 Debye because of mutual polarization in the longest c hain studied.