I happened to stumble upon something today that I thought was unusual. The problem is illustrated in a simple example: if you take the following basic cubic Si input file and, say, the Si.pz-vbc.UPF pseudopotential from the QE website and do a test for convergence with respect to ectutwfc.
If we look at the 4 bands that are calculated we see the following results: At ecutwfc=40 Ry: -5.1740 7.0370 7.0370 7.0370 ecutwfc=100Ry: -5.1746 7.0369 7.0369 7.0369 ecutwfc=200Ry: -5.1746 7.0369 7.0369 7.0369 Now try ecutwfc=2000Ry: -5.2862 6.9066 6.9066 10.2399 Now why do the values change? If I look at the output file for the 2000 Ry case I see that there is a negative starting charge: Initial potential from superposition of free atoms Check: negative starting charge= -0.057614 But still, why is this? The usual answer for a negative starting charge is to increase the wavefunction cutoff, although I suspect that's not the problem in this case..... So besides the using of a 2000 Ry cutoff for silicon, what else is wrong here? Best, Brad UC Berkeley &control prefix = 'si' calculation = 'scf' restart_mode = 'from_scratch' wf_collect = .false. tstress = .true. tprnfor = .true. outdir = './' wfcdir = './' pseudo_dir = './' / &system ibrav = 0 celldm(1) = 10.2612 nat = 2 ntyp = 1 nbnd = 4 ecutwfc = 2000.0 / &electrons electron_maxstep = 100 conv_thr = 1.0d-10 mixing_beta = 0.7 diago_full_acc = .true. / CELL_PARAMETERS cubic 0.000000000 0.500000000 0.500000000 0.500000000 0.000000000 0.500000000 0.500000000 0.500000000 0.000000000 ATOMIC_SPECIES Si 28.086 Si.pz-vbc.UPF ATOMIC_POSITIONS crystal Si -0.125000000 -0.125000000 -0.125000000 Si 0.125000000 0.125000000 0.125000000 K_POINTS automatic 1 1 1 0 0 0 -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20100122/728285a7/attachment.htm