  In order to quantitatively study the accuracy of the unconditionally stable
coarsening algorithms, we calculate the Fourier space multi step error on the
order parameter field by explicitly distinguishing the analytic time $\tau$ and
the algorithmic time $t$. The calculation determines the error in the order
parameter and the scaled correlations. This error contributes a correction term
in the analytic time step, which is crucial in understanding the accuracy in
unconditionally stable coarsening algorithms.
