[Pw_forum] How to show more digits in the results?

[Axel Kohlmeyer]

On Mon, Sep 1, 2014 at 2:32 PM, Mike Marchywka <marchywka at hotmail.com> wrote:

[...]

>> For that, you do _not_ need more digits for exactly the reason I gave
>> you. If you output more digits, you are just adding random numbers.
>> Literally.
>
> I suppose in general there could be offset problems with energies and you are adding
> some large fixed amount to a small change of interest. Or, you could want to examine

that would only make matters worse, since in that case you lose
additional digits of precision due to the lower "density" of floating
point numbers at large absolute values.

> the numerical issues.

what would be the point of computing a number with a significantly
large systematic error with extreme numerical precision?
not to mention that only because we treat atom cores as point
particles, we can specify their positions with high precision.

aiming to do accurate calculations is commendable, but one has to put
things into perspective and be able to tell numerical precision from
actual accuracy of the calculation. a lot of calculations we do look
much more precise than what their accuracy is.

axel.

>>
>>>
>>> Please let me know if you or anyone know how to change the code
>> showing more digits in pw.x.
>>
>> Many people know (including me). But why should anybody tell you, if
>> what you want is a stupid thing? Have you talked about this with your
>> adviser?
>>
>> In general, please spent a little time learning about the systematic
>> errors of DFT, the numerical precision of floating point numbers, the
>> accuracy of a self-consistent calculation and error propagation.
>>
>> You should quickly see the folly of your request.
>>
>>> Best regards
>>> Haibei
>>>
>>>
>>> On Sun, Aug 31, 2014 at 1:51 PM, Axel Kohlmeyer
>> <akohlmey at gmail.com<mailto:akohlmey at gmail.com>> wrote:
>>>>
>>>>
>>>> On Aug 30, 2014 10:57 PM, "Haibei Huang"
>> <haibeih at student.unimelb.edu.au<mailto:haibeih at student.unimelb.edu.au>>
>> wrote:
>>>>>
>>>>> Dear All,
>>>>>
>>>>> I am aware of that the QE will only show at most 8 digits after
>> the decimal point in the output file, while more digits are needed in
>> my calculation.
>>>>>
>>>>> Could any of you provide a way to increase the number digits after
>> the decimal point?
>>>>
>>>> What is the point of using more digits, when the error is larger?
>> You could just add random digits and have the same effect.
>>>>
>>>>>
>>>>> Thanks for your time!
>>>>> Looking forward to your reply!
>>>>> Regards
>>>>> Haibei
>>>>>
>>>>>
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-- 
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.