window method and all methods with fuller treatment of correlation showed median discard times at greater than 50 % of the total time. This indicates that when large and rapidly decaying initial transients dominate slowly increasing random errors, the 50 % time rule may be effective as a simulation-stopping criterion (especially when used with generalised MSER methods which more fully account for autocorrelation). This appears to be the case for Oliveira et al.'s data. However, our results suggest that this is unlikely to be the case for most free energy calculations.