An asymmetric model for water structure

Most classical computer simulations of water use an effective potential which reflects the local symmetry of the water molecule's nuclei. This in turn gives rise to a symmetric distribution of neighbouring molecules on average. Recent data from x-ray absorption spectroscopy however are interpreted as implying that there is local asymmetry in water structure with some strong and some weak hydrogen bonds. Here, existing neutron and x-ray diffraction data are interpreted with empirical potential structure refinement using an asymmetric water potential. It is found that quite accurate representations of the diffraction data are possible by this means. This does not prove that the asymmetric model is correct, nor that the symmetric model is incorrect, but that x-ray and neutron diffraction data on water are rather insensitive to these details of the water interaction potential.