Hi Guodong Yu C11 C12 C22,etc are the stiffness coefficient of the stiffness matrix say [C] . Which elastic constant are you referring is not clear ? Can you explain ?

Inverse the stiffness matrix, you will get the compliance matrix, then you can pick out the corresponding Young's modulus from the diagonals.

Manish Kumar Dubey Sorry I am not professional about elastic properties of materials. I read some paper about the deformation potential scattering. They show me "the elastic constant for longitudinal vibration", so I am not sure the corresponding term. Thanks

Wenbin Yu Thanks. Do you mean the Young's modulus is the same as 'elastic constant for longitudinal vibration'? Could you give me some text or papers about this. Thanks. I must be very sure about this. By the way, do you mean just do the verse of the elastic constant matrix, which was calculated by VASP.

Dear Dr Guodong Yu,

As Prof. Wenbin Yu suggested, if you are looking for the elastic constants you just need to calculate the inverse of the stiffness matrix [C], i.e. [S]=[C]-1 and obtain the elastic constants by direct comparison. As for example, E11=1/S11 and so forth. You can refer to the book of Prof. S.W. Tsai, Prof J.N. Reddy and many others where you'll find out a deeper explanation, if needed. For you reference I have attached herein a brief explanation,taken from my PhD Thesis, on how to obtain the elastic constants for a 2D anisotropic material. The coefficients eta you will see are the coefficients of mutual influence.

Regards