[DFTB-Plus-User] Failure in diagonalisation

[Zack Terranova]

I understand that DFTB+ will fail with this error: "ERROR!
-> Failure in diagonalisation routine zhegvd, non-positive definite overlap! Minor XXX responsible"
when two atoms become too close together, or are on top of each other.  But why are they failing in the middle of the geometry optimization job, for example at Geom step 53 or 82?

Thanks in advance!

Zack