Tutorial for Voids tool (1) Select (2) Depress
Voids are not a big computation. Start from a small radius like this, and then improve by trial-and-error.
Only change from defaults Depress
The voids larger than 1.4Å radius appear as small turquoise spheres with radius the radius of the void minus the threshold, here 1.4Å. In diamond, there are two kinds of voids, one quite obvious, and a smaller one at the centre of slightly kinked C hexagons. Those are stable positions where interstitial atoms could fit. Trial-and-error search for disappearance of a void is an easy way to measure its diameter.
End of Voids tool tutorial