The nature of interactions between clusters of Mg and Zn with HCN from symmetry-adapted perturbation theory based of DFT.

The donor-acceptor complexes HCN-Mg(n) and HCN-Zn(n) (n=1,...,4), which were recently detected in helium nanodroplet infrared spectroscopy experiments by Miller and co-workers [Science 292, 481 (2001); J. Phys. Chem. A 110, 5620 (2006)] are investigated by the symmetry-adapted perturbation theory based on the density functional monomer description [SAPT(DFT)]. The interaction energy components, such as the electrostatic, exchange, induction, and dispersion, are calculated as a function of the metal cluster size. We find that the donor-acceptor interactions manifest themselves by the large induction and dispersion interactions, which counteract the unusually large exchange repulsion. The dependence of the components on the clusters size n follows different patterns in the complexes of magnesium and zinc. In HCN-Mg(n) the induction effect increases in magnitude much faster than the dispersion effect. In HCN-Zn(n) there is a slight decrease in both dispersion and induction terms between n=2 and n=3. Then dispersion rises faster than induction between n=3 and n=4. The exchange effects are also much different in both types of complexes. The first-order exchange energy rises much faster with n in the magnesium complexes than in the zinc complexes. Furthermore, in the latter there is a significant drop in the exchange energy between n=2 and n=3. The second-order exchange effects tend to quench a larger percentage of the induction and dispersion contributions in the Mg(n) complexes than in Zn(n). These different patterns of the interaction energy variations with n are related to the different nature of nonadditive effects in the neat metal clusters.