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

[Mike Marchywka]

________________________________
> Date: Sun, 31 Aug 2014 21:41:11 -0400 
> From: akohlmey at gmail.com 
> To: pw_forum at pwscf.org 
> Subject: Re: [Pw_forum] How to show more digits in the results? 
> 
> 
> On Sep 1, 2014 1:16 AM, "Haibei Huang" 
> <haibeih at student.unimelb.edu.au<mailto:haibeih at student.unimelb.edu.au>> 
> wrote: 
>> 
>> Dear Axel, 
>> 
>> Thanks for your reply. 
>> 
>> The truth is that I was trying to diagonalize a matrix made of a 
> serials of Energies obtained by pw.x, while some of elements in the 
> matrix is quite small (e.g. 10^-7 or 10-^8). Thus, I would like to know 
> how many digits I could trust in Energies. Or at least I should keep 
> the Energies and these small elements in the same scale, say 10^-8. 
> 
> 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
the numerical issues. 

> 
>> 
>> 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 
>>>> 
>>>> 
>>>> _______________________________________________ 
>>>> Pw_forum mailing list 
>>>> Pw_forum at pwscf.org<mailto:Pw_forum at pwscf.org> 
>>>> http://pwscf.org/mailman/listinfo/pw_forum 
>>> 
>>> 
>>> _______________________________________________ 
>>> Pw_forum mailing list 
>>> Pw_forum at pwscf.org<mailto:Pw_forum at pwscf.org> 
>>> http://pwscf.org/mailman/listinfo/pw_forum 
>> 
>> 
>> 
>> _______________________________________________ 
>> Pw_forum mailing list 
>> Pw_forum at pwscf.org<mailto:Pw_forum at pwscf.org> 
>> http://pwscf.org/mailman/listinfo/pw_forum 
> 
> _______________________________________________ Pw_forum mailing list 
> Pw_forum at pwscf.org http://pwscf.org/mailman/listinfo/pw_forum