  The vibrational frequency of hydrogen molecules has been observed to increase
strongly with He concentration in helium hydrogen fluid mixtures. This has been
associated with He-H interactions, either directly through chemical bonding, or
indirectly through increased local pressure. Here, we demonstrate that the
increase in the Raman frequency of the hydrogen molecule vibron is due to the
number of H$_2$ molecules participating in the mode. There is no chemical
bonding between He and H$_2$, helium acts only to separate the molecules. The
variety of possible environments for H$_2$ gives rise to many Raman active
modes, which causes broadening the vibron band. As the Raman active modes tend
to be the lower frequency vibrons, these effects work together to produce the
majority of the shift seen in experiment. We used Density Functional Theory
(DFT) methods in both solid and fluid phases to demonstrate this effect. DFT
also reveals that the pressure in these H$_2$-He mixture is primarily due to
quantum nuclear effects, again the weak chemical bonding makes it a secondary
effect.
