Explicit hydration of ammonium ion by correlated methods employing molecular tailoring approach

Explicit hydration studies of ions require accurate estimation of interaction energies. This work explores the explicit hydration of the ammonium ion (NH₄⁺) employing Møller–Plesset second order (MP2) perturbation theory, an accurate yet relatively less expensive correlated method. Several initial geometries of NH₄⁺(H₂O)_n (n = 4 to 13) clusters are subjected to MP2 level geometry optimisation with correlation consistent aug-cc-pVDZ (aVDZ) basis set. For large clusters (viz. n > 8), molecular tailoring approach (MTA) is used for single point energy evaluation at MP2/aVTZ level for the estimation of MP2 level binding energies (BEs) at complete basis set (CBS) limit. The minimal nature of the clusters upto n ≤ 8 is confirmed by performing vibrational frequency calculations at MP2/aVDZ level of theory, whereas for larger clusters (9 ≤ n ≤ 13) such calculations are effected via grafted MTA (GMTA) method. The zero point energy (ZPE) corrections are done for all the isomers lying within 1 kcal/mol of the lowest energy one. The resulting frequencies in N–H region (2900–3500 cm⁻¹) and in O–H stretching region (3300–3900 cm⁻¹) are in found to be in excellent agreement with the available experimental findings for 4 ≤ n ≤ 13. Furthermore, GMTA is also applied for calculating the BEs of these clusters at coupled cluster singles and doubles with perturbative triples (CCSD(T)) level of theory with aVDZ basis set. This work thus represents an art of the possible on contemporary multi-core computers for studying explicit molecular hydration at correlated level theories.