How to use the Buckingham potentials

[lml... at gmail.com]

I notice that there is a Buck4range, close to the format of
Buckingham, but it is for different range. How can I use the
Buckingham potentials as the following format (gromacs has this kind
of Buckingham potentials)?

V(r) = A* EXP(-B*r)-C/r^6

By the way, I want to use the the unit in my input,  kcal/
mol*Angstrom, for C instead of the default angstrom*K_e. How can I
input for this?  I try [angstromkcalmol], but it fails.

Thanks a lot!

Limin