[Pw_forum] about ecutwfc convergence for total energy

[James Mao]

Hi all,

 

I am a little bit confused about ecutwfc convergence. I am working on
calculating 
NMR of a crystal structure. Before that I need test SCF energy convergence. 
However I noticed the total energy is not convergent with respect to
ecutwfc. 

It reached minimum at ecutwfc=90 then began oscillating up and down. 

 

Ecutwfc  Total Energy

70           -3249.733804

80           -3250.674250

90           -3250.827326

100         -3250.661301

 

120         -3250.346091

 

160         -3250.582989

 

200         -3250.395401

 

I think higher ecutwfc means more basis functions, therefore should always
result
in a lower total energy. If I am wrong, should I keep increasing ecutwfc?

 

The PP I used is (pbe-tm-new-gipaw-dc.UPF), the atoms include C, O, N, F, S,
H, P.

Since PP is NC, I left ecutrho default. And since the cell is pretty big
(a~10, b~16, C~15), 
I only calculated at gamma point.

 

Any hint is helpful! Thanks.

 

Best,

James Mao

----------------------------------

Postdoc, Department of Chemistry, University of Pittsburgh

 

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