B5. I’ve done VMC + varmin + VMC, and the energy (or variance) of the second VMC calculation is greater than the final ‘mean energy’ reported during varmin. Does this mean that the optimization has failed?

Short answer: No, you must compare the VMC energies and variances and ignore what varmin tells you. Optimization fails if the energy of the second VMC run is significantly greater than that of the first VMC run.

Long answer: The ‘unreweighted variance’ is a good target function to minimize to lower the TRUE energy (that of the later VMC run), but the values it takes are of no physical significance. This often applies to the ‘reweighted variance’ as well. Notice that the initial value of the two target functions must be the true variance of the first VMC run (or the true variance of a subset of the configurations if you set vmc_nconfig_write < vmc_nstep, which is usually the case). The same applies to the ‘mean energy’ reported in varmin.

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Category: B: Using CASINO

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