  Simulations are widely used to study nucleation in first order phase
transitions due to the fact that they have access to the relevant length and
time scales. However, simulations face the problem that nucleation is an
activated process. Therefore, rare event simulation techniques are needed to
promote the formation of the critical nucleus. The Seeding method, where the
simulations are started with the nucleus already formed, has proven quite
useful in efficiently providing estimates of the nucleation rate for a wide
range of orders of magnitude. So far, Seeding has been employed in the NPT
ensemble, where the nucleus either grows or redissolves. Thus, several
trajectories have to be run in order to find the thermodynamic conditions that
make the seeded nucleus critical. Moreover, the nucleus lifetime is short and
the statistics for obtaining its properties is consequently poor. To deal with
these shortcomings we extend the Seeding method to the NVT ensemble. We focus
on the problem of bubble nucleation in a mestastable Lennard Jones fluid. We
show that, in the NVT ensemble, it is possible to equilibrate and stabilise
critical bubbles for a long time. The nucleation rate inferred from NVT-Seeding
is fully consistent with that coming from NPT-Seeding. The former is quite
suitable to obtain the nucleation rate along isotherms, whereas the latter is
preferable if the dependence of the rate with temperature at constant pressure
is required. Care should be taken with finite size effects when using
NVT-Seeding. Further work is required to extend NVT seeding to other sorts of
phase transitions.
