Quantifying multiple-body interaction terms in H-bonded HCN chains with many-body perturbation/coupled-cluster theories

Many-body perturbation/coupled-cluster calculations have been carried out to investigate the multiple-body energy terms and their contribution to the interaction energy of linear (HCN)(N) chains. All minimum energy geometries of the clusters (N = 2-7) are obtained at the second-order many-body perturbation (MP2) levels of theory. Electron correlation and cooperative effects in the C-H...N hydrogen bonds are also quantitatively characterized during the aggregation process. It is found that the two- and three-body terms account for nearly all of the total interaction energy, but all high-body terms increase with the size of the cluster. Detailed numerical values are given for all the many-body contributions of the (HCN)(N) chains. Electron correlation effects are found to be important for the two- and three-body terms but have decreased importance for the higher-body terms. Cooperative effects are also investigated for the binding energy and dipole moment. The dipole moments of the HCN oligomers are larger than the sum of the individual monomers with differences ranging between 12% (N = 2) and 28% (N = 7). The limiting values for the binding energy and dipole moment of (HCN)(N), per monomer, corresponding to very large N values, are estimated to be 22.9 kJ/mol and 3.87 D, per monomer, respectively. These results correspond to cooperative contributions of 5.8 kJ/mol to the energy, and 1.0 D to the dipole moment. (C) 2003 American Institute of Physics.