[QE-users] Convergence of Ecutwfc and Ecutrho for Doublet Occupation Scheme

[Brendan Smith]

Hi QE Experts,

I have a system whose ground state is a doublet ( 1 spin up electron, in
this case ). I am using ultrasoft PPs. The system is a 3x3x3 super cell of
diamond with a single NV center.

My strategy for converging Ecutwfc and Ecutrho follows the prescription
laid out by Stephano in previous exchanges on the forum. It involves two
steps:

1) Using the default Ecutrho = 4*Ecutwfc, I converge the total force of my
cell w.r.t Ecutwfc.

2) Then, using the converged value of Ecutho, I scale back Ecutwfc to see
how low I can get it without coming out of my tolerance window, which is
1mRy in the total force computed using the converged parameters.

When I make my unit cell charged (therefore a triplet ground state [2 spin
up electrons] ) - I get a great convergence.

However, when doing step1 using my charge neutral (doublet) ground state, I
obtain the following for step 1 in the convergence procedure:

20.0000    0.238517
30.0000    0.273459
40.0000    0.272399
50.0000    0.270205
60.0000    0.269809
70.0000    0.269807
80.0000    0.274589
90.0000    0.274698
100.0000  0.270163
110.0000  0.270168
120.0000  0.270176
130.0000  0.270245
140.0000  0.274755
150.0000  0.270246

The left column is Ecut (Ry), the right column is Total Force (Ry/Bohr).
One can notice "spikes", for example, at Ecut = 140, and E cut = 80-90 Ry.

Can such a thing happen for a particular difficult occupation scheme? I
suspect that I am oscillating between two solutions, and that using Ecut 50
(therefore Ecutrho = 200 Ry) works here. Then, using Ecutrho = 200, I can
most likely get the Ecutwfc back to ~ 35 or so.

Thank you for your time,
Best,
Brendan A. Smith

4th year graduate student studying computational chemistry at the State
University of New York at Buffalo
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