The so-called 'Force Theorem' derived by Andersen (1975) describes how the change in the total energy of an electron system can be calculated to first order in a virtual displacement. It is demonstrated here, how this theorem can be applied in calculation of elastic constants of crystals although this in fact requires calculation of total-energy changes accurate to second order in the distortion parameter.
 

Solid State Communications, 49 701-5, 1984.