  Equilibrium properties of hydrogen-helium mixtures under thermodynamic
conditions found in the interior of giant gas planets are studied by means of
density functional theory molecular dynamics simulations. Special emphasis is
placed on the molecular-to-atomic transition in the fluid phase of hydrogen in
the presence of helium. Helium has a substantial influence on the stability of
hydrogen molecules. The molecular bond is strengthened and its length is
shortened as a result of the increased localization of the electron charge
around the helium atoms, which leads to more stable hydrogen molecules compared
to pure hydrogen for the same thermodynamic conditions. The {\it ab initio}
treatment of the mixture enables us to investigate the structure of the liquid
and to discuss hydrogen-hydrogen, helium-helium, and hydrogen-helium
correlations on the basis of pair correlation functions.
