[DFTB-Plus-User] Failure in diagonalisation routine with atoms not too close

[Mathias Jørgensen]

Dear DFTB+ community

I have been using the DFTB+ for optimising Mo2C clusters (Mo12C6) without problems. I use it for optimising initial random structures for a genetic algorithm (with atoms positioned randomly within a cell with some minimum interatomic distance). 

Recently I moved to a larger system, MoC24H18, and I very often get the following error:

"Failure in diagonalisation routine dsygvd, non-positive definite overlap! Minor 7688 responsible."

where “7688” differs depending on the initial structure.

For smaller systems the error still occurs but is much less frequent. 
I learned from older posts on this error that the problem often has to do with atoms getting too close or other non-physical systems. In my case, however, atoms getting too close it not the problem - I checked a lot of the geometries and ran tests with large initial minimum interatomic distances. Also, I learned that 7688 refers to orbital 7688, but I certainly don’t have 7688 orbitals in my system, Mo(d)C(p)H(s). The error is consistent with respect to initial atomic configuration and it happens at geometry step 0.

What could be wrong?

Best,
Mathias

*********
Mathias Jørgensen
Phd. Stud.
Interdisciplinary Nanoscience Center (iNANO), research group of prof. Bjørk Hammer
Aarhus University
Denmark
email: mj at inano.au.dk