[Pw_forum] What happens at REALLY large ectuwfc?

[Stefano Baroni]

I would be surprised if roundoff errors in FFT should be blamed for the reported behavior
SB

On Jan 25, 2010, at 11:34 AM, O. Baris Malcioglu wrote:

> Dear Brad,
> 
> Apart from what is already mentioned, another contribution to error
> you are mentioning might be due to computational/numeric issues.
> 
> Run-of-the-mill FFT algorithms has  an error upperbound that goes like
> O(sqrt(N)). Even in the best case (least performing) scenarios the
> error upper bound is O(log(N)).
> 
> These errors are due to numeric precision employed, and mostly bound
> by hardware (unless you compromise performance significantly).
> 
> In general, in any calculation that involves computers, in a graph
> that shows the "error" as a function of some convergent parameter, I
> would expect three regions , first the error diminishes due to
> numerical reasons, then there is a flat region, then roundoff errors
> begin to dominate, and error picks up again.
> 
> If interested, please have a look at the (rather rusty but still
> valid) concept "Machine constant"
> 
> Best,
> Baris.
> 
> 
> 
> 2010/1/25 Paolo Giannozzi <giannozz at democritos.it>:
>> Brad Malone wrote:
>>> I originally saw this problem with espresso-4.1.1 in a
>>> different system (AlAs on a 2x2x2 shifted grid).
>> 
>> start with 'random' or 'atomic+random' initial wavefunctions
>> (startingwfc='...'). I didn't find anything wrong with very
>> high cutoffs. Occasionally you can end up in the wrong ground
>> state, though, especially in highly symmetric cases like the
>> Si example you sent.
>> 
>> "Ghost states" are another story. It may happen that a
>> (nonlocal, separable) pseudopotential has a localized
>> spurious state that doesn't show up (i.e. it is not
>> occupied) at low cutoff, but it is occupied at higher
>> cutoff. None of the PP on the QE web site should have
>> such states, though.
>> 
>> If you have evidence of further problems, please provide
>> a test case so that it can be reproduced. In any case,
>> thank you for finding a bug in the cvs version!
>> 
>> P.
>> ---
>> Paolo Giannozzi, Democritos and University of Udine, Italy
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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