  The geometric--arithmetic indices are widely considered in the chemical graph
theory in the last decade. The reason of introducing new indices is to gain
prediction of target properties of considered molecules that is better than the
prediction obtained by already known indices. In the case of the fifth
geometric--arithmetic index hitherto no ability of prediction of some molecule
property has been considered.
  In this paper we investigate correlations between the fifth
geometric--arithmetic index and some other degree--based topological indices on
the family of octane isomers and polyaromatic hydrocarbons. Since a very good
correlation is established with the well known atom--bond connectivity index
for polyaromatic hydrocarbons and then for the alkane series, the relation
between the heat of formation and the fifth geometric--arithmetic index is
examined and a good correlation is confirmed in that case as well.
