Known issues

Known issues

• Thermalization time of worm code
  As described in the paper, in the thermalization process, we estimate the relative volumes of the worm spaces iteratively using the Wang-Landau algorithm. The estimates may converge slowly, depending on the details of the impurity model. Once we fail to obtain the converged results, we will some weird behavior. For example, we may see some error message like "Configuration space for **** was not visited" or "No observable found with the name: ***". There are three possible solutions:
  (1) Run the code longer so that the estimates converge well.
  (2) Run the code with multiple CPU cores so that the estimates are averaged over multiple processes at the end of the thermalization process.
  (3) Turn off the measurement of unnecessary observables to improve the speed. They will help.

• Instability of worm measurement of equal-time single-particle Green's function
  When the mean number of electrons is very close to 0, the measurement of equal-time single-particle/two-particle Green's functions becomes unstable. You may see error messages something like "Configuration space for **** was not visited in measurement steps. Thermalization time may be too short." This means that you failed to obtain converged estimates of the volumes of their configuration spaces in the given thermalization time. This is because the volumes of their configuration spaces vanish.
  
  In practical, one may increase the thermalization time, or (if it does not help) change the chemical potential. This is a technical issue in the current implementation. We will solve this issue in the next minor update.