[DFTB-Plus-User] Hydrogen deviation term

[Zack Terranova]

Hey all, 

I came across a paper (The SCC-DFTB method and its application to biological systems) Theor Chem Acc (2006) 116: 316-325 that talks about the pros and cons of DFTB and different ways of correcting some approximations.  Equation 14 has me stumped; basically we can improve accuracy by optimizing the gamma function (Gamma = 1/R -S) where S is an exponentially decaying short range element.  For hydrogen it is possible to throw in this additional term f(αβ) that affects S.  I've searched, and I cannot find what f(αβ) should be!
I'll take any ideas anyone has!  Thank you all!

Zack Terranova
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