  The design of moderators and cold sources of neutrons is a key point in
research-reactor physics, requiring extensive knowledge of the scattering
properties of very important light molecular liquids such as methane, hydrogen
and their deuterated counterparts. Inelastic scattering measurements constitute
the basic source of such information but are difficult to perform, the more so
when high accuracy is required, and additional experimental information is
scarce. The need of data covering as large as possible portions of the
kinematic Q-E plane thus pushes towards the use of computable models, validated
by testing them, mainly, against integral quantities (either known from theory
or measured) such as spectral moments and total cross section data. A few
recent experiments demonstrated that, at least for the self contribution, which
dominates in the incoherent scattering case of hydrogen, accurate calculations
can be performed by means of quantum simulations of the velocity
autocorrelation function. This method is shown here to be by far superior to
the use of standard analytical models devised, although rather cleverly, for
generic classical samples. The neutron dynamic structure factor (and
consequently the well-known S({\alpha},{\beta}) of parahydrogen and deuterium,
suitable for use in packages like NJOY, are given and shown to agree very well
with total cross section measurements and expected quantum properties.
