Model dependence of quantum isotope effects in liquid water

Path-integral molecular-dynamics simulations have been carried out for liquid water at room temperature using three different potential functions: ST2, SPC, and TIP4P. Quantum isotope effects on the liquid structure are computed in order to examine the dependence of these structural changes on the model used, and a comparison is made to corresponding measurements. The SPC model is found to be in excellent agreement with experimental results; for this model the oxygen–oxygen pair distribution function shows a change in shape and slight shift to smaller distance of the second-neighbor peak when going from D2O to H2O. In contrast, the other two models both show a distinct outward shift of this peak. This difference between models can be attributed to subtle differences in the direct interaction energies of second-nearest-neighbor molecules.