  Grand canonical molecular dynamics (GCMD) is applied to the nucleation
process in a metastable phase near the spinodal, where nucleation occurs almost
instantaneously and is limited to a very short time interval. With a variant of
Maxwell's demon, proposed by McDonald [Am. J. Phys. 31: 31 (1963)], all nuclei
exceeding a specified size are removed. In such a steady-state simulation, the
nucleation process is sampled over an arbitrary timespan and all properties of
the metastable state, including the nucleation rate, can be obtained with an
increased precision.
  As an example, a series of GCMD simulations with McDonald's demon is carried
out for homogeneous vapor to liquid nucleation of the truncated-shifted
Lennard-Jones (tsLJ) fluid, covering the entire relevant temperature range. The
results are in agreement with direct non-equilibrium MD simulation in the
canonical ensemble. It is confirmed for supersaturated vapors of the tsLJ fluid
that the classical nucleation theory underpredicts the nucleation rate by two
orders of magnitude.
