  Condensation of H 2 in the interstellar medium (ISM) has long been seen as a
possibility, either by deposition on dust grains or thanks to a phase
transition combined with self-gravity. H 2 condensation might explain the
observed low efficiency of star formation and might help to hide baryons in
spiral galaxies.
  Our aim is to quantify the solid fraction of H 2 in the ISM due to a phase
transition including self-gravity for different densities and temperatures in
order to use the results in more complex simulations of the ISM as subgrid
physics.
  We used molecular dynamics simulations of fluids at different temperatures
and densities to study the formation of solids. Once the simulations reached a
steady state, we calculated the solid mass fraction, energy increase, and
timescales. By determining the power laws measured over several orders of
magnitude, we extrapolated to lower densities the higher density fluids that
can be simulated with current computers.
  The solid fraction and energy increase of fluids in a phase transition are
above 0.1 and do not follow a power law. Fluids out of a phase transition are
still forming a small amount of solids due to chance encounters of molecules.
The solid mass fraction and energy increase of these fluids are linearly
dependent on density and can easily be extrapolated. The timescale is below one
second, the condensation can be considered instantaneous.
  The presence of solid H 2 grains has important dynamic implications on the
ISM as they may be the building blocks for larger solid bodies when gravity is
included. We provide the solid mass fraction, energy increase, and timescales
for high density fluids and extrapolation laws for lower densities.
