While doing NSCF, I decided to rerun it at higher
nbnd values to incorporated more unoccupied states while plotting PDOS. But the Fermi energy gets changed with each different
nbnd. What is the reason for this?
&CONTROL calculation='nscf' title='graphene' prefix='graphene' restart_mode='from_scratch' outdir='../tmp' wf_collect=.true. pseudo_dir='~/work/pot/' / &SYSTEM ibrav = 4, a = 14.58 , c= 20, nat = 3 ntyp = 2 vdw_corr = 'dft-d3' ecutwfc = 50.0 , ecutrho = 250.0 , occupations='smearing' smearing='gaussian' degauss=0.001 nbnd=30 / &ELECTRONS electron_maxstep = 200, conv_thr = 1.0d-10 , / &IONS ion_dynamics = 'bfgs' / ATOMIC_SPECIES Na 22.989 Na.pbe-spnl-kjpaw_psl.1.0.0.UPF S 32.065 S.pbe-n-kjpaw_psl.1.0.0.UPF ATOMIC_POSITIONS (angstrom) S 0.3560556497 2.9232871880 0.0000000000 Na 2.0049069168 0.9398232344 0.0000000000 Na -1.8523938267 1.8291379398 0.0000000000 K_POINTS automatic 6 6 1 0 0 0
Here, I tried nbnd=16 which gave Fermi energy 1.8 eV, and then nbnd=30 which gave 2.8 eV.
Also, the system being optimized is a molecule though this change in Fermi energy was observed in crystals too. I have only tried it in insulators(with band gap) which could be the reason for this since the Fermi energy is not well defined for insulators (Other than saying that it's in the middle of the bandgap). But then how do various papers report the DOS of insulators with F.E. as the origin and compare them?
Update : this problem doesn't exist for scf calculations! repeated the same process by using scf and yielded the same Fermi energy. Would the problem be because initially I ran scf without smearing and nscf with smearing?