Introduction Extensive studies of dissociation of acids, bases, and salts have been carried out to understand solvation phenomena.1-3 These phenomena are strongly involved in H-bonding and proton transfer. For a better understanding of H-bonding and proton transfer, it would be of interest to investigate such phenomena in the ionized state, which can be easily observable in the stratosphere. We are particularly interested in the hydration of the ammonia cation, namely, (NH3 · · ·H2O)+. Ammoinum cluster cations (NH3)n + and their hydrated clusters [(NH3)n[(H2O)m]+ were experimentally produced under special conditions in the gas phase.4 A few theoretical investigations for ammonia, water, and ammonia-water cluster cations were performed.5-8 The ammonia dimer cation and the ammonia-water cation were experimentally discussed.9 For the ammonia dimer cation, the disproportionated ionic structure (NH4 + · · ·NH2) was predicted to be more stable than the hydrazine-like structure (H3N· · ·NH3).6 In previous work, one of us reported that for the water dimer cation at high levels of ab initio theory, the H3O+ · · ·OH structure is much more stable than the H2O· · ·OH2 structure.10 Here, we investigate the structures, energetics, and spectra of the ammonia-water cation (NH3H2O)+ and the ammonia dimer cation (NH3)2 + using high levels of ab initio theory. We compare the DFT, MP2, and CCSD(T) results. Serious failures for most DFT calculations are found, except for few functionals. Calculation Methods Before calculating the ionized structure of the ammonia-water cluster and the ammonia dimer, we need to calculate their neutral structure as the reference system. Since these structures are already well-known,11,12 we calculated their structures and energies at the CCSD(T)/aVQZ//CCSD(T)/aVDZ level of theory. For the ionized structures, various structures of the ammonia-water cation and the ammonia dimer cation are optimized by using DFT methods with various functionals. For the DFT calculations, we employ various functionals, Becke’s exchange and Lee-Yang-Parr correlation functionals (BLYP),13 Becke’s exchange and Perdew-Wang correlation functionals (BPW91),14 Handy’s family functional including gradient-corrected correlation (HCTH407),15 the local spin density approximation, Vosko-Wilk-Nusair correlation and Slater exchange functionals (LSDA: SVWN),16 semiempiricalcorrection to BLYP for dispersion (BLYP-D),17 Tao-Perdew- Staroverov-Scuseria exchange and τ-dependent gradientcorrected functionals (TPSS),18 Becke’s three-parameters for exchange and Lee-Yang-Parr correlation functionals (B3LYP),19 Zhao and Truhlar’s parametrized exchange and correlation hybrid meta-GGA M05-2X,20 Perdew-Burke- E zerhof hybrid functional (PBE1PBE),21 modified Perdew- Wang one-parameter model/modified Perdew-Wang and Becke one-parameter model for kinetics (MPW1K/MPWB1K),22,23 Becke’s half HF-LSDA (Hartree-Fock Local Spin Density Approximation) exchange and Lee-Yang-Parr correlation functionals (BH&H),24 and Becke’s half HF-LSDA-Becke exchange and Lee-Yang-Parr correlation functionals (BH&HLYP).25 For these DFT calculations, we used the 6-311++G** basis set.26 Then, as noted in the water dimer cation,10 we also find that in the ammonia-water cation and the ammonia dimer cation, DFT/MPW1K and DFT/ BH&HLYP are reliable, while others give seriously wrong energy values, as compared with the CCSD(T)/CBS values. Here, we compare the DFT, MP2, and CCSD(T) results using the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets27 (which will be denoted as aVDZ, aVTZ, and aVQZ, respectively). The CBS limit interaction energies were obtained with the extrapolation * To whom correspondence should be addressed. E-mail: abcd0lhm@ postech.ac.kr. J. Phys. Chem. A 2009, 113, 6859–6864 6859 10.1021/jp903093a CCC: $40.75 2009 American Chemical Society Published on Web 06/02/2009 Downloaded by AUSTRIA CONSORTIA on July 6, 2009 Published on June 2, 2009 on http://pubs.acs.org | doi: 10.1021/jp903093a scheme utilizing the fact that the electron correlation error is proportional to N-3 for the aug-cc-pVNZ basis set (N ) 2: D; N ) 3: T; N ) 4: Q) [ΔECBS ) (ΔENN3 - ΔEN-1(N - 1)3)/(N3 - (N - 1)3)].28 Here, the CBS energies, which would give the most reliable values in these calculations, were obtained based on the aVTZ and aVQZ results. In this way, we could compare their CBS values and find the inherent errors in the DFT and MP2 results. For the DFT and MP2 calculations using the aVDZ and aVTZ basis sets and the CCSD(T) calculations using the aVDZ basis set, the geometries were fully optimized, and frequency calculations were also carried out. A larger grid size (99, 974) than the ultrafine grid (99, 590) was employed to eliminate the imaginary frequencies for the DFT calculations. All of the optimizations were done by minimizing the total energy without any symmetry constraints. In the DFT and MP2 calculations using the aVQZ basis set, the corresponding aVTZ geometries were used, and the aVTZ frequencies were employed to obtain the zero-point energies (ZPEs) and thermal energies, while in the CCSD(T) calculations using the aVTZ and aVQZ basis sets, the corresponding aVDZ geometries were used, and the aVDZ frequencies were employed to obtain the ZPEs and thermal energies. For the basis set, the 1s orbitals of oxygen atoms were frozen in the correlation calculations. All of the “d” and “f” orbitals used here are the spherical harmonic basis functions (5d and 7f). For the ionic structure, the basis set superposition energy (BSSE) correction can be made. However, in the nonionic structure, the positive charge is almost equally distributed in two monomer species so that the BSSE correction cannot be made properly. In order to compare the two structures at equal conditions, it is better not to make the BSSE corrections. Thus, the BSSE corrections are not considered in this system. We calculated the ZPE’s uncorrected total energy (ΔEe) at the equilibrium states of the Bo -Oppenheimer potential surfaces and the ZPE-corrected total energy (ΔE0). The enthalpy/ free-energy changes (ΔHr/ΔGr) at room temperature and 1 atm were obtained using the frequency calculations. All of the calculations were carried out by using the Gaussian 03 suite of programs.29 The BLYP-D calculations were done by using ORCA program30 and the M05-2X calculations by Q-Chem program.31 The molecular structures were drawn using the POSMOL package