Mass-density and Phonon-frequency Relaxation Dynamics of Water and Ice   at Cooling
  Coulomb repulsion between the bonding electron pair in the H-O covalent bond (denoted by "-") and the nonbonding electron pair of O (":") and the specific-heat disparity between the O:H and the H-O segments of the entire hydrogen bond (O:H-O) are shown to determine the O:H-O bond angle-length-stiffness relaxation dynamics and the density anomalies of water and ice. The bonding part with relatively lower specific-heat is more easily activated by cooling, which serves as the "master" and contracts, while forcing the "slave" with higher specific-heat to elongate (via Coulomb repulsion) by different amounts. In the liquid and solid phases, the O:H van der Waals bond serves as the master and becomes significantly shorter and stiffer while the H-O bond becomes slightly longer and softer (phonon frequency is a measure of bond stiffness), resulting in an O:H-O cooling contraction and the seemingly "regular" process of cooling densification. In the water-ice transition phase, the master and the slave swap roles, thus resulting in an O:H-O elongation and volume expansion during freezing. In ice, the O--O distance is longer than it is in water, resulting in a lower density, so that ice floats. 
Chang Q. Sun
physics.chem-ph cond-mat.soft