| Summary: | Equilibrium geometries, interaction energies and harmonic frequencies of (NH3)n (n = 2 5) and NH4+(NH3)n (n = 2 5) were computed using correlated electronic calculations (MP2) in conjunction with aug-cc-pVXZ (X=D, T, Q) basis sets and the Counterpoise procedure. The zero-point energy (ZPE) on the relative stability of the clusters was estimated using harmonic frequencies. For both pure and protonated ammonia clusters we found that using basis set superposition error (BSSE) corrected forces or freezing the monomer structure to its gas phase geometry had only a weak impact on the energetics and structural properties of the clusters. For pure ammonia clusters, (NH3)n (n = 2 5), we found that low lying isomers for (NH and (Nfls have similar binding energies, perhaps suggesting the presence of a very smooth energy landscape. The harmonic frequencies highlighted the presence of vibrational fingerprints for the presence of double acceptor ammonia molecules. In addition, many-body effects for n = 2 4 were investigated we found the 3-body effects to account for 10-15% of the total interaction energy and 4-body effects to be negligible. Under these premises, a model pair interaction fitted to ab initio data for rigid ammonia molecules was developed. It was extended with a description of polarisation effects, introduced by using a noniterative form of the charge-on-spring model, the latter accounting for more than 95% of the dipole induction energy and of the increased molecular dipole. This model was used to optimise putative global minima for (NHs)n (n = 3 20) the structure and energetics of the clusters with n = 2 5 were found to be in good agreement with previous ab initio results. For larger isomers our model predicts larger binding energies than previous analytical surfaces, and also predicts a reorganisation of the energy ranking and a different global minimum structure. For protonated ammonia clusters, we have found two general types of isomeric structures, globular and linear, the former showing larger binding energies. Harmonic frequencies reveal that the signature of these clusters is given mainly by NH4+. In agree ment with the literature we also found that higher frequencies for the N-H vibrational modes of the NH4+ are seen upon increasing cluster size. Finally, the vaporisation en ergy computed in this work compares well with previous theoretical and experimental data.
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