Reblock convergence with large number of walkers
Posted: Mon Oct 06, 2014 5:01 pm
Dear CASINO Users,
As I read from the manual, number of walkers for statistical dmc calculation should be around or more than 1000. However, if I want to use more walkers, say 20000 for example, I was able to get smaller variances by using less number of steps, like one-tenth steps. Although the variance of energy is 8*10^-4 Ha for nearly 100 electron system at 10000 statistical steps (for 20000 walkers), error of standard mean is still around 30 % of standard mean.
It seems advantageous to use large number of walkers if you want to use large number of processors for a short time, but reblocking error requires a some amount of steps to make accurate prediction of the error(it seems like I need to use more than 10000 steps no matter what number of walkers I use for my system).Since number of walkers should not be larger than number of processors, this brings a limitation on number of processors that can be used. Please correct me if I am wrong until here. That is why, I am wondering if there is a good strategy to decrease correlation length. Performing many VMC steps and printing very few of them can be useful I guess as the first step, but I can't think of anything else. Any idea on minimizing CPU run time while minimizing the wall time could be very useful.
Thanks,
Kayahan
As I read from the manual, number of walkers for statistical dmc calculation should be around or more than 1000. However, if I want to use more walkers, say 20000 for example, I was able to get smaller variances by using less number of steps, like one-tenth steps. Although the variance of energy is 8*10^-4 Ha for nearly 100 electron system at 10000 statistical steps (for 20000 walkers), error of standard mean is still around 30 % of standard mean.
It seems advantageous to use large number of walkers if you want to use large number of processors for a short time, but reblocking error requires a some amount of steps to make accurate prediction of the error(it seems like I need to use more than 10000 steps no matter what number of walkers I use for my system).Since number of walkers should not be larger than number of processors, this brings a limitation on number of processors that can be used. Please correct me if I am wrong until here. That is why, I am wondering if there is a good strategy to decrease correlation length. Performing many VMC steps and printing very few of them can be useful I guess as the first step, but I can't think of anything else. Any idea on minimizing CPU run time while minimizing the wall time could be very useful.
Thanks,
Kayahan