In order to model an interstitial trapped in a metal lattice, and the associated incorporation energy, which should be used - "relax" or "vc-relax"? I want to understand the lattice expansion from the interstitial, and that tells me vc-relax, while i realize that an interstitial introduction would involve some lattice strain which would suggest relax.
I was using the energy difference $E_{\rm intAinM} = E(M+A)-(E(M)+E(A))$ to calculate interstitial incorporation energy from both, and they are not the same as one would imagine.
I would love you hear some suggestions or insights into this.