When DFTB+ with the version >=21.1 is used, there is a problem about the overlap matrix calculation.

[DaehoHan]

I tested DFTB+ v21.1, 21.2 and 22.1, and they failed to give overlap matrix elements of a doubled molecule when two molecular images consisting of it are too close.

*** Geometry step: 0
WARNING!
-> Atoms 1 and 11 too close to each other! (dist= 0.492994E-02)
WARNING!
-> Atoms 3 and 13 too close to each other! (dist= 0.722506E-03)
WARNING!
-> Atoms 5 and 15 too close to each other! (dist= 0.486282E-02)
WARNING!
-> Atoms 8 and 18 too close to each other! (dist= 0.803423E-02)
WARNING!
-> Atoms 9 and 19 too close to each other! (dist= 0.222407E-02)
WARNING!
-> Atoms 10 and 20 too close to each other! (dist= 0.558394E-02)
ERROR!
-> Atoms 1 and 11 too close together

Now, the openmp implementation of DFTB+ v20.1 only seems to work.
When using the higher versions, one perhaps need to edit their codes.