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["D. R. Hamann" <drhamann@mat-simresearch.com>]

Hi m_mousavi,

In general I agree with Pierre-Matthieu's comments below, but would like 
to clarify the issue a bit more.

For most purposes, the unit cell should be fully relaxed so that the 
stresses are effectively zero before doing a calculation of the elastic 
tensor using the response-function (or any) method.  For some purposes, 
however (such as geophysics) you may want the elastic constants under 
high-stress conditions.  It is OK to use the response-function 
calculation in this case, but the results must be corrected as discussed 
in doc/theory/elasticity-oganov.pdf, using Eq. (17) in that document to 
correspond to the most widely used definition of elastic constants under 
stress.  Since recent Abinit releases output the stress tensor in a DDB 
file, in principle this correction could be computed as part of the 
anaddb post-processing needed for almost all elasticity calculations, 
but this has not yet been implemented.

Best regards,
Don  Hamann



Anglade Pierre-Matthieu wrote:
> Hi,
>
> If you use the experimental data you may run into a few problems
> related to a potentialy high pressure.
> Suppose the PSP predict the lattice parameter with an error of ~ 1%
> with respect to experiment and your bulk modulus is 100 GPa, You will
> evaluate elastic constant and other properties under a pressure of 1
> GPa~ 10^4 atm.
> Also, the computation of elastic constant by means of response
> function may not allow you  to use pressure. Or may not handle this
> very well. Yet if you want try, and many will be intersted if you can
> come back and share your findings here.
> Endly, it is sometime faster and safer to obtain the elastic constant
> by "manual" finite difference method. At least, you can use it to
> check your results.
>
> regards
>
> PMA
>
> On 9/26/07, m_mousavi1980@yahoo.com <m_mousavi1980@yahoo.com> wrote:
>   
> >  Hi
> >
> >  I want to run elastic lesson for zno
> >
> >  can I use experimental latice parameter for input acell.
> >
> >  How can I understand my out put for telast_1 IS true.
> >
> >  regard
> >
> >
> >     
>
>
>   

--
D. R. Hamann
Mat-Sim Research LLC    | Deptartment of Physics
P.O. Box 742            |  and Astronomy
Murray Hill, NJ 07974   | Rutgers University
phone: 908-370-8079     | 732-445-4381

email: drhamann@mat-simresearch.com