Correlated sampling, variational Monte

A well-known solution for this problem is correlated sampling. In this case all evaluations of finite differences are done within one QMC calculation. This approach reduces the statistical error and circumvents the computational costs of doing several quantum Monte Carlo calculations. So far correlated sampling was only investigated in a closed unbiased form for variational Monte Carlo. The VMC correlated sampling calculation involves wave functions of the reference structure ij/ and the perturbed structure... 

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