About the pressure

[thangckt]

Dear Prof. Jochym,

Thank you for sharing your code to compute elastic constants.

May I have a question related to the usage of pressure in stress-strain relationship, at this line

https://github.com/jochym/Elastic/blob/820d9764303ad4df068d26e97c1c358d92fbab7d/elastic/elastic.py#L530C19-L530C59

• Why we need pressure here, and normalize again after that (line 544)
• Can we just simply use stress difference between deformed and reference structures instead, say sl.append(deform_cryst.get_stress()-ref_cryst.get_stress())

Thank you so much for your help.

[jochym]

In high symmetry structures, with atoms in high-symmetry positions, there is no difference. However, when atoms are not in such positions under pressure, you need to find the new equilibrium positions in the deformed cell. This may be insignificant, but may be also quite important. It depends.

[thangckt]

Dear Prof @jochym

Thank you so much for your explaination. However, I am still not clear how to address the pressure contribution in the stress-strain equation. I can not find this in standard textbooks about elasticity.

Can you kindly introduce some publications which mentioned this dependence to refer? Thank you so much for your help.