182 P.Entel et al. Neutron diffraction scattering dsta Car-Parrinello MD 2 0 0 4567 8 r(A) Fig.9.1.The oxygen-oxygen (top),oxygen-hydrogen (middle)and hydrogen- hydrogen (bottom)radial distribution functions obtained from a Car-Parrinello molecular dynamics simulation of a system of 64 H2O molecules(solid lines)[9.27] compared to results of neutron diffraction scattering 9.26(dashed lines).The total simulation time was 11 ps and the average ionic temperature was 307 K.The figure was adapted from [9.27]. Of all the RDFs,goo(r)is perhaps the most informative.Calculation of the coordination number defined by Tmin Nc=4πp drr2goo(r), (9.9) (where rmin is the location of the first minimum in goo(r))gives the number of nearest neighbor water molecules.The recent X-ray data give Ne =4.7 9.25 while the CP simulations yield an average number of H2O molecules in the first coordination shell of about 4.0.A coordination number N<5 indicates that liquid water preserves much of its tetrahedral,ice-like structuring,while differences from the ice-like structuring can be found from the hydrogen-bonding patterns,see,for example,9.33.The ratio of the dis- tances of the first two peak positions in goo(r)is about 1.64 close to the ideal182 P. Entel et al. 0 1 2 3 gOO(r) Neutron diffraction scattering data Car-Parrinello MD 0 1 2 3 gOH(r) 0 12345678 r (Å) 0 1 2 3 gHH(r) Fig. 9.1. The oxygen-oxygen (top), oxygen-hydrogen (middle) and hydrogen￾hydrogen (bottom) radial distribution functions obtained from a Car-Parrinello molecular dynamics simulation of a system of 64 H2O molecules (solid lines) [9.27] compared to results of neutron diffraction scattering [9.26] (dashed lines). The total simulation time was 11 ps and the average ionic temperature was 307 K. The figure was adapted from [9.27]. Of all the RDFs, gOO(r) is perhaps the most informative. Calculation of the coordination number defined by Nc = 4πρ r min 0 dr r2 gOO(r), (9.9) (where rmin is the location of the first minimum in gOO(r)) gives the number of nearest neighbor water molecules. The recent X-ray data give Nc = 4.7 [9.25] while the CP simulations yield an average number of H2O molecules in the first coordination shell of about 4.0. A coordination number Nc < 5 indicates that liquid water preserves much of its tetrahedral, ice-like structuring, while differences from the ice-like structuring can be found from the hydrogen-bonding patterns, see, for example, [9.33]. The ratio of the dis￾tances of the first two peak positions in gOO(r) is about 1.64 close to the ideal