In molecular dynamics
simulations in the NPT ensemble at constant
pressure, the size and shape of the periodic simulation box fluctuate
with time. For particle images far from the origin, the rescaling
of the box by the barostat results in unbounded position displacements.
Special care is thus required when a particle trajectory is unwrapped
from a projection into the central box under periodic boundary conditions
to a trajectory in full three-dimensional space, e.g., for the calculation
of translational diffusion coefficients. Here, we review and compare
different schemes in use for trajectory unwrapping. We also specify
the corresponding rewrapping schemes to put an unwrapped trajectory
back into the central box. On this basis, we then identify a scheme
for the calculation of diffusion coefficients from NPT simulations,
which is a primary application of trajectory unwrapping. In this scheme,
the wrapped and unwrapped trajectory are mutually consistent and their
statistical properties are preserved. We conclude with advice on best
practice for the consistent unwrapping of constant-pressure simulation
trajectories and the calculation of accurate translational diffusion
coefficients.