Cut-off problrm

simin pahlavi simin.... at
Tue Apr 24 11:16:54 UTC 2018

Hello Matteu,
I would like to thank you very much for your Helpful comments again.
Have a nice day

On Tuesday, April 24, 2018 at 12:42:37 PM UTC+2, Matt W wrote:
> 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.
> Matt
> 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
>>>     XC_SMOOTH_RHO NN50
>>>    &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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