I am testing the convergence of a few isolated systems and I find problems in converging systems containing very electronegative elements. I am varying ecut and acell only, pseudo is PBE and kpt is always 1.
Most of the molecules converge fine, but it becomes more difficult the more I move right in the periodic table.
BF4 and PF6 are the worst molecules, never converging.
BF3 and BCl3 show some non-converging case, but taking an high ecut (>20) and an high acell (>30) is enough.
N2 also presents some problems.
So, what is wrong with PBE+halogen atoms?
Here's an example of non converging system (BF4):
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#Definition of the unit cell acell 3*30 #Definition of the atom types ntypat 2 znucl 5 9 #Definition of the atoms natom 5 typat 1 2 2 2 2 xcart 0.0001148660 0.0000001274 0.0000948037 0.0000254663 0.0000002928 1.4168343567 1.3357270137 0.0000003076 -0.4722566817 -0.6679098939 -1.1565412448 -0.4723165605 -0.6679091494 1.1565405706 -0.4723160520 #Definition of the planewave basis set ecut 30 #Definition of the k-point grid kptopt 0 # Enter the k points manually nkpt 1 # Only one k point is needed for isolated system, kpt 0.0 0.0 0.0 # taken by default to be 0.0 0.0 0.0 #Definition of the SCF procedure nstep 300 # Maximal number of SCF cycles toldfe 1.0d-6 diemac 2.0