# [Pw_forum] about the precision of energy and some extend problem.

Stefano Baroni baroni at sissa.it
Thu Dec 13 13:02:53 CET 2007

```Dear all: two further comments.

1) I have not seen in detail the output of Xu Yuehua's calculations,
but I would like to point out that one may have negative frequencies
even if the atomic forces and crystal stress vanish. Negative
frequencies are a manifestation of a mechanical instability that may
occur even when the first derivatives of the energy with respect to
any possible mechanical distortion of the lattice vanish.

2) I am not myself a native English speaker, and I know by experience
how much it takes to write in a foreign language. I am also aware that
the difficulty of writing decent English depends a lot on one's own
mother tongue. However, I would like to kindly urge contributors to
this mailing list to read twice their posts and always ask themselves
very carefully if what they have written can be understood with a fair
investment of time and attention. Working on one's own way of
expressing oneself is a very important (and rewarding, rest assured!)
exercise that non-native English speakers should do.

Best wishes,
SB

On Dec 12, 2007, at 12:11 PM, Hande Ustunel wrote:

> Dear Xu Yuehua,
>
> You should fit your energies to a parabola and find the minimum of the
> fit. For a second order fit, your minimum is at about 2.83. 2.77 is
> just a
> local blip in your rather jumpy data. You cannot decide by looking
> at the
> individual energies because they do not follow a smooth function.
> But even
> for the "wrong" lattice constant, there's no physical reason why you
> should
> get negative frequencies if you have converged your electronic and
> atomic
> coordinates to high accuracy. I'm attaching the fit as an example.
>
> I would also suggest increasing your cutoff a little because your
> data, as
> I said, is a bit noisy near the minimum.
>
> Best,
> Hande
>
> On Wed, 12 Dec 2007, xu yuehua wrote:
>
>> hi ,uesers here :
>> i relax one crystal lattice vs energy:
>> i have compute several energy list :
>>
>> c          E(RY)
>> 2.795  -137.79266
>> 2.78   -137.79248
>> 2.77   -137.79233
>> 2.8     -137.7927
>> 2.81  -137.79275
>> 2.82  -137.79275
>> 2.83  -137.79278
>> 2.84  -137.79276
>> 2.85  -137.79279
>> 2.86  -137.79272
>> 2.87  -137.79265
>> 2.88  -137.79258
>> 2.9   -137.79229
>> 2.95  -137.79122
>> these energy points are smooth and  can be plotted  parabola.
>> the system has 12 atoms.the energy is different most at the third
>> digit
>> after radix point. so i think the average energy/atom is less 0.0001.
>>
>> i want to ask  the  within the precision of energy(my thinking is
>> based on
>> that :my above calculation is set ecutwfc:29, ecutrho:200,and the
>> criterion
>> is 35,350, the convergence is 0.0049345 ry.)
>> so this small difference of energy can skip,i.e we can choose 2.77
>> which is
>> regarded as the best structure?
>> as why i ask this question,for the the structure of 2.85 has two  big
>> negative frequencies  near -100cm-1, even if i add asr .
>> but 2.77 has no negative frequency after i add asr.
>>
>>
>>
>
> --
> Hande Ustunel
> Department of Physics
> Office 439
> Middle East Technical University
> Ankara 06531, Turkey
> Tel : +90 312 210 3264
> http://www.physics.metu.edu.tr/~hande
> <fit.eps>_______________________________________________
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

Please, if possible, don't  send me MS Word or PowerPoint attachments
Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html

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