[DFTB-Plus-User] Fitting repulsive potential with spline, information about technical details

[Riccardo Rozza]

Dear all,
I am a new DFTB+ user. I'm trying to understand how the spline 
describing the repulsive potential in .skf file is built.
In particular there are three things I do not understand. If I represent 
the spline in the form (as reported in dftb.org)


------------
Spline
a)    nInt cutoff nInt = number of intervals
b)    a1 a2 a3 coefficients of exp[-a1+a2] + a3, describing the 
potential if r < r0
c)    start (r0) end c0 c1 c2 c3                        (for each 
interval until nInt-1)
d)    start end (cutoff) c0 c1 c2 c3 c4 c5        (for the last interval)
------------


where c0 c1 c2 c3 are the spline coefficients in the point c) What is 
not clear to me is:
1) How do I found a1 a2 and a3?
2) Why do I need a 5th order spline for the last interval while only a 
3rd order one is used before?
3) How do I decide the number of intervals?
Any help would be greatly appreciated.

Best wishes,
Riccardo Rozza