[Pw_forum] ev.x gives wrong results

[Paolo Giannozzi]

Funny: one of the parameters of the fit is good for small unit cells, but
too small for larger cells. In line 162 of PW/tools/ev.f90, replace
      deltapar(1) = 0.1d0
with something larger:
      deltapar(1) = 1.0d0
should be sufficient. Thank you for noticing this

Paolo

On Tue, Jul 28, 2015 at 6:06 PM, anyy zsj <zsjanyy at gmail.com> wrote:

> This data set is actual calculated because I use a large supercell:
> 10.1      -48.570860572794118
> 10.3      -49.368013045588235
> 10.5      -49.714550792647060
> 10.7      -49.667482497058820
> 10.9      -49.305953287500003
> But the fitting results are not compatible.
>
> 2015-07-28 12:01 GMT-04:00 Paolo Giannozzi <p.giannozzi at gmail.com>:
>
>> 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
>>
>> _______________________________________________
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://pwscf.org/mailman/listinfo/pw_forum
>>
>
>
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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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