I relaxed Na2S2 molecule in QuantumEspresso and plotted the planar average of potential energy across z axis so as to find the Vacuum potential. This is the output I got : As you can see the vacuum potential is not constant making it impossible to calculate it. While following the same procedure for Na2S I got the following: as you can see the vacuum potential is a constant value. What could be a reason for what happened in the case of Na2S2? How can I solve it ?
The input file for the geometry optimization of Na2S2:
&CONTROL calculation='relax' 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 = 4 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) Na -2.1578688813 -0.0711007913 0.0959862818 S 0.0279507567 0.7976264059 1.1138133631 S -1.4993815540 2.3462755509 0.6717950344 Na 0.8998813708 2.9052562024 -0.0587466613 K_POINTS automatic 2 2 1 0 0 0