## Random UserWarning: unused option ['P_tol']

How do I use this algorithm? What does that parameter do?
xaver
Posts: 5
Joined: 23 Apr 2021, 07:51

### Random UserWarning: unused option ['P_tol']

I frequently and randomly get the warning UserWarning: unused option ['P_tol'] for config lanczos_params warnings.warn(msg.format(keys=sorted(unused), name=self.name)) when starting a dmrg run.

I get this warning independent of if I do, or if I do not use 'lanczos_params': {'P_tol': some integer,} in the dmrg_params.

And, strangely enough, if it is not a batch-run, and I see the warning to occur, and then stop and restart the dmrg run 'by hand', the warning frequently does not show up again.

Is this expected behavior, should I worry about it / dismiss it, can I stop it?
Johannes
By default, the option P_tol_to_trunc causes DMRG to set the P_tol parameter in the lanczos_params, which causes this warning in case DMRG actually does not use lanczos to diagonalize the effective Hamiltonian (it does exact diagonalizaiton for small enough chi where this is actually faster).