[DFTB-Plus-User] Question about SK files for pair of elements

[Reinaldo Pis Diez]

Dear Bálint and folks,

Due to a recent error when trying to run dftb+ with a very simple 
example, I've verified that in some SK sets some atoms pairs are 
described by exactly the same set, whereas others are not. To be 
clearer, the chalc-0-1 set has the following SK files

As-As-skf
As-H.skf
As-S.skf
S-As.skf
H-As.skf -> As-H.skf

in which one can note that H-As and As-H share the same SK file 
through a symbolic link, but As-S and S-As are different files 
(there is a lot of different numbers if you make a diff on them). 
The same happens in pbc-0-2, for example.
My question is, Why are they different? Shouldn't be the same file 
as X-Y should interact the same way as Y-X? Am I missing something?
Thanks in advance.
Regards,

Reinaldo