Manfred Hartl. 
On automorphism groups of torsion-free nilpotent groups. 

Abstract : Automorphism groups of finitely generated torsion-free 
nilpotent groups $G$ are determined in terms of
iterated group extensions with lifting obstructions. Both are 
calculated in terms of polynomial constructions in the sense of Passi; in
particular all terms arising are explicitly computable from a 
given presentation of $G$. For free nilpotent groups of a given 
class the results take a particularly simple form.