[Pw_forum] ev.x gives wrong results

[Paolo Giannozzi]

Please show me a set of data *produced by actual calculations* that ev.x
doesn't fit properly

paolo

On Tue, Jul 28, 2015 at 5:51 PM, anyy zsj <zsjanyy at gmail.com> wrote:

> 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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>>
>
>
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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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