  We consider the problem of estimating the error when solving a system of
differential algebraic equations. Richardson extrapolation is a classical
technique that can be used to judge when computational errors are irrelevant
and estimate the discretization error. We have simulated molecular dynamics
with constraints using the GROMACS library and found that the output is not
always amenable to Richardson extrapolation. We derive and illustrate
Richardson extrapolation using a variety of numerical experiments. We identify
two necessary conditions that are not always satisfied by the GROMACS library.
