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 
.