[Pw_forum] Convergence test

[Huiqun Zhou]

List users,

I'm testing convergence on ecutwfc for chromite using USPP. Below are
results of different ecutrho with different ecutwfc. I may be wrong, I think
I should watch the difference of total energy between successive runs till
it's less than 1-2 mRy. But the trend is not apparent in the results below
and it seems that the larger the ecutwfc, the lower the total energy.

I calculated E-V relations using two sets of ecutwfc and ecutrho:
for ecutwfc = 35 ry and ecutrho = 240 ry, a = 8.405, B0 = 172.8, B0' = 4.3
for ecutwfc = 55 ry and ecutrho = 250 ry, a = 8.407, B0 = 178.6, B0' = 3.6

There are no big differences in unit cell parameter and elastic properties.
So, my questions are
(1) usually, the ecutwfc of USPP should be around 35 ry or below, is my
  case an exception?
(2) how small the difference of total energy of successive runs should be
  in judging convergence against ecutwfc?

Thank you in advance.

Dr. Huiqun Zhou
@Earth Sciences, Nanjing University, China




ecutrho = 240 ry
ecutwfc                     total energy
35                     -1461.54573299 Ry
40                     -1461.64206824 Ry
45                     -1461.71978835 Ry
50                     -1461.76587314 Ry
55                     -1461.76316597 Ry
60                     -1461.76692396 Ry

ecutrho = 250 ry
40                     -1461.64351876 Ry
45                     -1461.70130510 Ry
50                     -1461.74724764 Ry
55                     -1461.76462943 Ry
60                     -1461.76838812 Ry

ecutrho = 280 ry
50                     -1461.74778158 Ry
55                     -1461.76516515 Ry
60                     -1461.76892327 Ry