Accurate Energies and Structures for Large Water Clusters Using the X3LYP Hybrid Density Functional

We predict structures and energies of water clusters containing up to 19 waters with X3LYP, an extended hybrid density functional designed to describe noncovalently bound systems as accurately as covalent systems. Our work establishes X3LYP as the most practical ab initio method today for calculating accurate water cluster structures and energies. We compare X3LYP/aug-cc-pVTZ energies to the most accurate theoretical values available (n ) 2-6, 8), MP2 with basis set superposition error (BSSE) corrections extrapolated to the complete basis set limit. Our energies match these reference energies remarkably well, with a root-meansquare difference of 0.1 kcal/mol/water. X3LYP also has ten times less BSSE than MP2 with similar basis sets, allowing one to neglect BSSE at moderate basis sizes. The net result is that X3LYP is ∼100 times faster than canonical MP2 for moderately sized water clusters.