# Relax vs VC-Relax for interstitial incorporation energy

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.

• By interstitial incorporation are you considering the interstitial entity to be an impurity? – Anoop A Nair Dec 1 '20 at 11:18
• Yes. As in a light element (C, H, N or O) in a metal lattice. – Dr.Viper Dec 2 '20 at 3:32