Hi,
till recently I though that checking the convergence of total energy vs
k-grid cutoff is enough.
However, now I've found that while total energy can be very well
converged, Fermi level position is not, and requires at least twice
denser k-grid (and ~4 times more time).
Here is my example for bulk GaAs:
kgrid Ef, eV k-pnts SCFtime forces Etot
cutoff
8 -5,3105 32 1 0,00509 -789,50149
10 -4,9698 -- -- 2,19E-4 -789,44567
12,2 -4,1639 108 -- 0,0052 -789,50521
16,287 -4,1839 256 1,37 0,0028 -789,50544
20,359 -4,7579 500 -- 1E-3 --
26,467 -5,1201 1183 3,139 2E-4 -789,5052
30,5 -4,9783 1800 -- 4E-5 -789,5054
32,575 -4,7802 2048 4,73 6E-6 -789,50545
40,7 -4,7676 4000 -- 1,28E-4
50 -5,1057 16 2,07E-4
As you can see, Fermi level varies in the range of 0.6 eV!!! (all the
bands don't shift). The middle of the gap is at -4,75
My questions are:
1. How Fermi level is calculated?
Is it just filling the available bands with a given amount of electrons
(based on calculated DOS on a given k-grid) after all calculations are
done? What smoothing of DOS is used then???
Ef doesn't converge actually.
If 1. is true then I can understand it - DOS shape changes quite
significantly and real convergence would be only when one gets all
possible k-points.
2. Since the total energy is calculated on the same k-grid, why it
doesn't show the same behavior? i.e why it is less sensitive than Ef?
Should one bother at all about Ef during geometry relaxation?
3. Does the filling of the bands affect the forces on atoms, and thus
explicitly affects total energy?
Imagine such a situation:
a molecule adsorbs to a dangling bond on a surface only if it is empty
or only partially filled,
if I set the Ef 0.5eV higher, I make the dangling bond completely filled
and moleculed would not adsorb at all,
i.e. we end up in a completely different geometry and total energy.
I will appreciate very much any comments or suggestions.
Sincerely,
Alexander.
