HpBaseWorkChain: Convergence error-handler

[t-reents]

If a HpCalculation doesn't converge, the parameter niter_max is increased. The error handler doubles the value per default. One of my examples was apparently hard to converge wherefore the error-handler was called multiple times. Based on what I stated before, per default, the value of niter_max will be increased to 800 in the 4th iteration (after calling the error-handler three times). QE has a hard-coded limit of 500 for this parameter which causes the calculation to fail (https://github.com/QEF/q-e/blob/e3968f1e75e8661110548601b5381f9e496e6930/HP/src/hpcom.f90#L91). This being said, I thought that it might be useful to take this into account.

I think there would be different options to handle this.

1. Limit niter_max to 500 but allow alpha_mix to be further decreased (I'm new to this type of calculations, so not a 100% sure whether this makes sense).
2. In case 500 would be exceeded, do the last run with niter_max=500 and stop the WorkChain afterwards, if this didn't resolve the problem.

One could also think about considering that users might compiled their own version and the maximum of 500 doesn't apply in those cases.

[bastonero]

Hi @t-reents, thanks for the excellent suggestions.
As the two issues are not interconnected, could you please open a new issue with the parallelization problem and keep this one only for the handler (and edit it correspondingly)?
So we can open specific PRs to solve them separately.

[t-reents]

For sure.