Cut-off problrm

Matt W mattwa... at
Tue Apr 24 10:42:37 UTC 2018

Personal opinion is that converging total energy is not very important in 
this case - it is normally just a rigid shift based on inaccuracy of the 
core part of the wavefunctions. I am still (again personally) not really a 
big fan of the smoothing, I'd rather have a slightly bigger cutoff but a 
not messed with functional.

I would look at forces on atoms and see how they converge and see how 
errors in forces propagate to energy conservation.


On Tuesday, April 24, 2018 at 10:24:34 AM UTC+1, simin pahlavi wrote:
> Hello Matteu 
> Thank you very much fir your helpful comment.
> As I am supposed to use this DFT calculations for every step of an ab 
> initio molecular dynamics, I wonder if I should
> go for the heavy smoothing and get reasonable forces or I should be worry 
> about convergence and cutoff dependence of minimization?
> I appreciate any reply in advance
> Simin
> On Monday, April 23, 2018 at 10:41:00 PM UTC+2, Matt W wrote:
>> Sorry, I posted too quickly without really looking at your setup.
>> The big problem for you is the XC_GRID settings. 
>>    &XC_GRID
>>    &END XC_GRID
>> You really do not want to be using heavy smoothing, especially for this 
>> purpose. These settings (see original paper) help get reasonable forces 
>> with a rather low cutoff, but as you increase the cutoff lead to you 
>> continuously redefining the XC functional you use (you are smoothing over 
>> different areas of real space), so they do not converge.
>> Try changing to the defaults
>>    &XC_GRID
>>     XC_DERIV PW
>>    &END XC_GRID
>> you should see at least a significant improvement.
>> Matt
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