  The relaxation dynamics in two van der Waals bonded and one hydrogen-bonding
molecular liquids is studied as a function of pressure and temperature by
incoherent neutron scattering using simultaneous dielectric spectroscopy. The
dynamics is studied in a range of alpha relaxation times from nano- to
milliseconds, primarily in the equilibrium liquid state. In this range we find
that isochronal superposition and density scaling work not only for the two van
der Waals liquids, but also for the hydrogen-bonding liquid, though the density
scaling exponent is much smaller for the latter. Density scaling and isochronal
superposition are seen to break down for intra-molecular dynamics when it is
separated in time from the $\alpha$-relaxation in close agreement with previous
observations from molecular dynamics simulations.
