In PWscf the forces are calculated from the Hellman-Feynman theorem (strictly true only at the stationary point) plus a correction that (approximately) accounts for the residual lack of self-consistency. As such when the correction is large compared to the Hellman-Feynman term one should be careful. In your case I would say that the forces are now probably very small and therefore the structure is reliable. I would say that the actual (small) value for the force are probably correct within a few percent since the correction term is (according to the value quote) of the order of 10 % of the total (and hopefully is not completely wrong). Hope this helps, stefano de Gironcoli -SISSA and DEMOCRITOS
Madhura Marathe wrote: > Dear all, > > During one of the ionic relaxation calculations, I got the error message, > "SCF correction compared to forces is too large, reduce conv_thr". I am > using conv_thr = 1.0d-8 which is generally sufficient, so I increased the > parameter upscale (from 10.0 to 100.0) in &IONS to reduce the the > threshold during relaxation. This lead to the convergence till the last > ionic iteration which was converged to sufficient accuracy, when again the > same error message occurred. The forces then are > Total force = 0.000136 Total SCF correction = 0.000016 > SCF correction compared to forces is too large, reduce conv_thr > > There has been a recent discussion on the topic. From that, I gathered > that the subsequent relaxations after this error message are not > reliable. > So my question is whether the forces are reliable in this last iteration? > Or do I need to further reduce the conv_thr and re-run the whole > calculation? > > Thanks for the help, > Madhura. > >
