Water clusters to nanodrops: a tight-binding density functional study.

We predict structures and energies of water clusters containing up to 100 waters with tight-binding density functional theory (DFTB). A per-hydrogen-bond energy correction is found to correct for systematic errors in the DFTB cluster energies. We compare the DFTB structures and energies to density functional theory (DFT) calculations and to the most accurate wave function theoretical (WFT) values available (ranging from coupled-cluster theory to second-order perturbation theory). After including the simple hydrogen bond correction, we achieve a root-mean-square difference of less than one kcal mol(-1) with the best estimates. As DFTB optimizations are orders of magnitude faster than DFT or canonical MP2, it is apparent that DFTB is a very practical method for calculating large water cluster structures and, with the hydrogen bond correction, also energies.