  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.
