A problem with Monte Carlo (MC) simulation is that the error decreases only as , so need to do 100 times more work to get one more significant figure accuracy. Unfortunately, things are even worse. Error is only for N independent measurements. But MC configurations are usually correlated --- depends on .
Measure the autocorrelation function (similar to correlation function except in computer time, not real space) for an operator (measurement) A:
is the (exponential) autocorrelation time, and is equivalent to the which measures equilibration time of the Markov process. Need to thermalize for time .