A COMBINED DENSITY-FUNCTIONAL THEORY MOLECULAR MECHANICS FORMALISM AND ITS APPLICATION TO SMALL WATER CLUSTERS

We present a study of the conformations of the water dimer, trimer and tetramer using a combined density functional theory/ molecular mechan ics method (DFT/MM). Both local density approximation (LDA) and non-lo cal density functional (NLDF) calculations are reported, employing two basis sets (polarized valence double zeta (DZVP) and augmented correl ation consistent polarized valence double zeta (aug-cc-pVDZ)). In comp aring the optimized geometries and the binding energy from the DFT/MM calculations with those derived from pure DFT and previous MP2 treatme nts, we find that an NLDF calculation is required to derive energetics which match the MP2 results. In all the cases studied, LDA is found t o overestimate the binding energy and underestimate the oxygen-oxygen bond lengths (R(O-O)) between two hydrogen-bonded water molecules. We attribute this to the importance of an NL description of hydrogen bond s, as previously found by other researchers. For the combined DFT/MM c alculations, the situation is more complicated. While the MM term is, of course, not sensitive to NLDF, the DFT component of a partitioned D FT/MM system exhibits the same sensitivity as the pure DFT to NLDF, wh ile the DFT/MM coupling term is less sensitive. The sensitivities of t he DFT and DFT/MM calculations to the basis set are similar to the NLD F case. In all cases, the aug-cc-DZVP gives longer R(O-O) values and h igher (less negative) binding energies than DZVP, but underestimates t he binding energy when the H bond(s) exist(s) in the DFT region. The D FT/MM coupling term is found to be dependent on the initial geometry t o some extent. In summary, the DFT/MM calculations with properly selec ted basis sets and NLDF give energetics and geometric information whic h compare favorably with MP2 results. (C) 1997 Elsevier Science B.V. ( C) 1997 Elsevier Science B.V.