Application of the SCC-DFTB method to H+(H2O)6, H+(H2O)21, and H+(H2O)22.

The low-lying potential energy minima of the H(+)(H(2)O)(n), n = 6, 21, and 22, protonated water clusters have been investigated using two versions of the self-consistent-charge density-functional tight-binding plus dispersion (SCC-DFTB+D) electronic structure methods. The relative energies of different isomers calculated using the SCC-DFTB+D methods are compared with the results of DFT and MP2 calculations. This comparison reveals that for H(+)(H(2)O)(6) the SCC-DFTB+D method with H-bonding and third-order corrections more closely reproduces the results of the MP2 calculations, whereas for the n = 21 and 22 clusters, the uncorrected SCC-DFTB+D method performs better. Both versions of the SCC-DFTB+D method are found to be biased toward Zundel structures.