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

[Axel Kohlmeyer]

On Sep 1, 2014 1:16 AM, "Haibei Huang" <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.

>
> 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>
wrote:
>>
>>
>> On Aug 30, 2014 10:57 PM, "Haibei Huang" <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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>>
>>
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