I am working with a BCC molybdenum supercell (54 atoms) and trying to model a self-interstitial atom (SIA), specifically a ⟨111⟩ crowdion configuration. I want to add extra atom into supercell but in 111 direction.

I understand that a crowdion is not a single interstitial atom but a collective displacement of atoms along the ⟨111⟩ direction. However, I am facing confusion in:

1. How to correctly construct a physically valid crowdion structure (without atom overlap) in a BCC Mo lattice
2. Whether the crowdion should be introduced as an extra atom or as a displacement of an existing atomic chain
3. How this defect differs from tetrahedral and octahedral interstitials in practice

Additionally, is there any Perl script to generate such a defect in a supercell, or is manual construction the recommended approach?