[Pw_forum] ev.x gives wrong results

[anyy zsj]

Thank you very much!
Sure it is angstrom. My concern is that with the same data, ev.x and the
manual fitting give different results as shown above. So I believe
something is there.
The discrepancy is gone if I use a small lattice parameter. For example, if
I scale the data by 1/3, both of them give the same results. So what's
going on?

2015-07-28 11:44 GMT-04:00 Paolo Giannozzi <p.giannozzi at gmail.com>:

> On Tue, Jul 28, 2015 at 5:26 PM, anyy zsj <zsjanyy at gmail.com> wrote:
>
>
>>    1.4989396432   -6.0466776328
>>    1.5430233198   -6.0795705718
>>    1.5879629631   -6.0993455319
>>    1.6804430727   -5.9859764266
>> The first column is volume and the second energy with Ry. After ev.x I
>> input Ang, noncubic, 4, it gives me a results of 0.40 for minimal volume.
>> It is certainly wrong.
>>
>
> of course it is: ev.x assumes that the input data corresponds to a
> physical EOS. Your data isn't physical: the volume is way too small
>
>>
>> Another example for the below data:
>> 10.1      -48.570860572794118
>> 10.3      -49.368013045588235
>> 10.5      -49.714550792647060
>> 10.7      -49.667482497058820
>> 10.9      -49.305953287500003
>> The first column is lattice constant with angstrom and the second energy
>> with Ry.
>>
> After ev.x I input Ang, sc, 4, it gives me 10.50369, while I get a
>> different value 10.57517 by fitting to the formula in PRB 28,5480 (1983)
>> of Murnaghan's equation.:
>> http://gilgamesh.cheme.cmu.edu/doc/software/jacapo/appendices/appendix-eos.html
>>
>
>
> sure that the first column is in angstrom and not a.u.? This is what I get
> assuming it is in a.u.:
>
>  # equation of state: murnaghan.        chisq = 0.5402D-05
> # a0 = 10.5740 a.u., k0 =13776 kbar, dk0 =  4.17 d2k0 =  0.000 emin =
> -49.73649
> # a0 =  5.59554 Ang, k0 =1377.7 GPa,  V0 = ******* (a.u.)^3,  V0 =175.197
> A^3
>
> note the small value of the |chi|^2 (chisq) of the fit
>
> Paolo
>
> I have tested many groups of data but the discrepancy is there for those
>> large lattice parameters. Does ev.x use another equation or something else
>> I have missing? Thank you very much.
>>
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>
>
> --
> Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
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