Bernal–Fowler rules
From Glossary of Meteorology
The set of rules describing the arrangement of hydrogen atoms in an "ideal" ice crystal.
These rules are 1) Each water molecule is oriented such that its two hydrogen atoms are directed approximately toward two of the four surrounding oxygen atoms (arranged almost in a tetrahedron); 2) only one hydrogen atom is present on each O-O linkage; and 3) each oxygen atom has two nearest neighboring hydrogen atoms such that the water molecule structure is preserved. Violations of these rules lead to structural defects in ice, responsible, among other things, for its electrical conductivity and its long (10-4 s) electrical relaxation time.
