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dear sir/madam,

Is it possible to calculate the strain of cell parameter without changing the cell volume ? anybody who has experience in strain calculation, i need your help ?

I'm not exactly sure what you mean.

But if you are interested in the calculation of the stress tensor then it is calculated for ISIF>2 in the INCAR file for a given cell (ISIF=2 is the default). Here is the corresponding wiki page wiki/index.php/ISIF.

If you look for elastic constants then please set IBRION=6 and ISIF>=3:
wiki/index.php/Phonons_from_finite_differences#Output

Thank you so much.

during tensile and compress strain (it may be strain to a, b or c), the cell volume shrinks. My question is can we keep the original cell volume while calculating the strain ?

I think you have to do it manually, since strain is an arbitrary deformation chosen by the user. That means if you shrink one volume by a given amount you have to make the remaining cell parameters larger so that the overall volume stays the same.

Do you want to describe the behaviour of your material of interest under stress and strain?
If yes, then usually the elastic constants of the material are used to characterize that and you should carry out the above described "ISIF>=3 and IBRION=6" calculations.

Unstrainend cell parameter (volume of cell : 61.4)

4.6487519736122431 0.0000000000000000 0.0000000000000000
-0.0000000000000000 4.6487519736122431 0.0000000000000000
0.0000000000000000 0.0000000000000000 2.8662250282021859
strained parameter (strained to the C direction) (volume of cell : 68.1359)

4.6487519736122431 0.0000000000000000 0.0000000000000000
-0.0000000000000000 4.6487519736122431 0.0000000000000000
0.0000000000000000 0.0000000000000000 3.152847531

Thank you for your reply. Yes, I want to study the behaviour of material of interest under stress and strain. The volume of cell changed after strained, is it possible to keep the original volume while strain ?

I used "ISIF>=3 and IBRION=6" calculations, but the volume changed.

IBRION=6 uses trial moves that may change the volume, but the elastic constants are calculated for the original volume.

Thank you so much.