  A simple model of the molecular crystal of $N$ atoms as a statistical mixture
in real space of $NX$ atoms in excited and $N(1-X)$ atoms in well localized
ground state is considered. The phase coherence of the atomic wave functions is
suppose to be absent. A bond energy of crystal is supposed to be a result of
the pair interaction of $NX$ excited atoms. These molecular type pair
excitations do not interact one with another before the metallization, and do
not contribute to the pressure. Nevertheless, the pressure of such kind of
crystals is determined by the interatomic distances, and by the binding energy
of pairs. The possibility of the insulator-superconductor transition of such a
``gas'' of $NX/2$ pairs, ``dissolved'' among $N(1-X)$ atoms in ground state is
discussed. This kind of transition is supposed to occur in the oxigen $O_2$, in
the sulphur $S$, and, possibly, in the xenon $Xe$ crystals under pressure. The
same kind of transition is likely to take place in HTSC materials,
metal-ammonia and hydrogen-palladium solutions under normal conditions, due to
similarity of some of their properties with the corresponding ones of molecular
crystals.
