elastic constant calculation in ABACUS

[hongriTianqi]

Background

Elastic constants are important to analyse the mechanical properties of solids. ABACUS can calculate stress in optimization, can elastic constants and Young's modulus be given for an optimized structure in ABACUS?

Describe the solution you'd like

Add the function of calculating elastic constants in ABACUS.

[mohanchen]

I think this is a great idea, but you should clarify how many detailed steps we need to implement this new feature.