[Wien] Vol optimization for BaRuO3, propose to change eosfit

[Peter Blaha]

Thank you very much for your improvements.

I agree that better starting values will help the fit and thus take your 
suggestions for Murnaghan and Birch-Murnaghan.

However, for EOS2, the values of "x" have nothing to do with E0, V0, B, 
BP and thus using your starting values may be good in one case, but not 
good in others.

For instance for our famous "TiC" example, the fitted values are:

Equation of state: EOS2 (PRB52,8064)        info           2
  a,b,c,d     -1778.379016       -30.538896      -132.038868 
727.308860

Even -1778.379016 is quite far from E0 ( -1783.962) and the others can 
be large positive or negative in various examples.

Best regards
Peter

On 02/27/2015 10:56 AM, Lyudmila Dobysheva wrote:
> On 26.02.2015 00:28, delamora wrote:
>> Indeed there is no fit for the EOS2, Murnaghan, but for the
>> Birch-Murnaghan there is a fit.
>>   V0,B(GPa),BP,E0            NaN            NaN            NaN
>>   Equation of state: Murnaghan                info           5
>>   E=E0+[B*V/BP*(1/(BP-1)*(V0/V)**BP +1)-B*V0/(BP-1)]/14703.6
>> V0,B(GPa),BP,E0            NaN            NaN
>> NaN               NaN
>>       390.4272   -25794.173431    -0.000730          NaN            NaN
>>   Equation of state: Birch-Murnaghan                info           5
>>   E = E0 + 9/16*(B/14703.6)*V0*[(eta**2-1)**3*BP +
>> (eta**2-1)**2*(6-4*eta**2)]
>>          --> eta = (V0/V)**(1/3)
>>   V0,B(GPa),BP,E0        74.4783    -14614.5174         5.9286
>> -25780.948961
>
> The fit for the Birch-Murnaghan is also not good: gives bad V0, though
> on the plot it is clearly seen by eye.
> I have tried a fit of Murnaghan through simply gnuplot, and it gave
> rather good answer:
> V0= 454.289          +/- 7.335        (1.615%)
> B = 97.7233          +/- 25.87        (26.48%)
> BP= 7.63145          +/- 0.8111       (10.63%)
> E0= -25794.2         +/- 0.002309     (8.952e-06%)
>
> I checked in the eosfit a choice of starting parameters and it is really
> not good.
> In your case you have rather large scope in volume deviations, and I
> think that when initial parameters for fitting are too far from the
> minimum point it gives error. I have also met sometimes with such cases
> of unclear refuse to work.
>
> So I'd like to suggest an amendment to eosfit: it is better to take the
> minimun energy and the correspondent volume as initials.
> I am sending the changed version of the program in attachment, I think
> it should be more stable.
> The amendments suggested:
> lyu at trusty>diff eosfit.f eosfit.f-old
> 48,60c48,51
> < !        FINDING THE MINIMUM ENERGY
> <     Emin=e(1)
> <     kmin=1
> <     do 1222 k=2,i-1
> <     if (e(k).gt.Emin) goto 1222
> <     Emin=e(k)
> <     kmin=k
>
> <  1222    continue
> < !
> <       x(1)=e(kmin)
> <       x(2)=vol(kmin)
> <       x(3)=100.d0
> <       x(4)=5.d0
> ---
>  >       x(1)=e(1)
>  >       x(2)=1.d0
>  >       x(3)=-10.d0
>  >       x(4)=100.d0
> 103,104c94,95
> <       x(1)=e(kmin)
> <       x(2)=vol(kmin)
> ---
>  >       x(1)=emurna(3)
>  >       x(2)=vol(3)
> 175,176c166,167
> <       x(1)=e(kmin)
> <       x(2)=vol(kmin)
> ---
>  >       x(1)=ebm(3)
>  >       x(2)=vol(3)
>
> Now it gives Murnaghan:
>  From gnuplot      454.289    97.7233   7.63145  -25794.2
> V0,B(GPa),BP,E0   454.2473   97.8633   7.6275   -25794.221591
> and Birch-Murnaghan:
> V0,B(GPa),BP,E0   469.9829   31.3139  27.4110   -25794.223651
>
> Best regards
>    Lyudmila Dobysheva
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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
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