  The structure around oxygen atoms of four silicate liquids (silica, rhyolite,
a model basalt and enstatite) is evaluated by ab initio molecular dynamics
simulation. Thanks to the use of maximally localized Wannier orbitals to
represent the electronic ground state of the simulated system, one is able to
quantify the redistribution of electronic density around oxygen atoms as a
function of the cationic environment and melt composition. It is shown that the
structure of the melt in the immediate vicinity of the oxygen atoms modulates
the distribution of the Wannier orbitals associated with oxygen atoms. In
particular the evaluation of the distances between the oxygen-core and the
orbital Wannier centers and their evolution with the nature of the cation
indicates that the Al-O bond in silicate melts is certainly less covalent than
the Si-O bond while for the series Mg-O, Ca-O, Na-O and K-O the covalent
character of the M-O bond diminishes rapidly to the benefit of the ionic
character. Furthermore it is found that the distribution of the oxygen dipole
moment coming from the electronic polarization is only weakly dependent on the
melt composition, a finding which could explain why some empirical force fields
can exhibit a high degree of transferability with melt composition.
