Bond Angle-Length and Density Oscillation

36 Thermally Mven Density and Phonon-Stiffness Oscillation 

As seen in Fig. 36.5b, the 6 angle widening contributes to the volume change. In the liquid phase I, the mean 6 valued at 160° remains almost constant. However, the snapshots of the MD trajectory in Fig. 36.5c and the MD movie [45] (Appendix A4-1) show that the V-shaped H-H-O molecules remain intact at 

The calculated evolution of the 0-0 distance shown in Fig. 36.5d agrees well in trend with the measured density in the concerned temperate range [6]. In ice, the 0-0 distance is always longer than that in water, and hence, ice floats. The 0-0 length relaxation dominates the density evolution at transition while the mixed-phase configuration plays a complementary role. 

The MD-movie (Appendix A4-3) shows that in the liquid phase, the H and the O attract each other between the 0 H, but the 0-0 repulsion prevents this occurrence. The intact O-H-O motifs are moving restlessly because of the high fluctuation and frequent switching of the 0 H interactions. Furthermore, the cooperative and ojfj shifts provide further evidence for the persistence of the Coulomb repulsion between the bonding and the non-bonding electron pairs in liquid. Therefore, the H2O in the bulk form of liquid could possess the tetrahedrally coordinated structures with thermal fluctuation [4]. 

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