A series of self-consistent field, molecular orbital cluster calculations has been conducted to model the influence of hydrogen on certain structural units in the grain boundaries of nickel. The structural units considered were two Bernal polyhedra, namely the capped trigonal prism and the Archimedean antiprism. Other calculations were performed to simulate interstitial hydrogen and atomic hydrogen chemisorbed on the (111) surface. It was found that hydrogen causes local expansion as an interstitial impurity, in rough quantitative agreement with experiments on dilute solid solutions of hydrogen in nickel. In contrast, it was found that a single hydrogen atom causes a shrinkage of about 1.7% in the bond lengths of the capped trigonal prism and about 2.8% in those of the Archimedean antiprism. Similarly, a hydrogen atom chemisorbed above a nickel atom causes the nearest (111) surface bonds to shorten by about 1.7%. The shrinkage induced in the two Bernal polyhedra appears to be correlated to the low electron density at their centres, and the overall reduction of charge in the vicinity of certain nickel-nickel bonds upon the inclusion of the hydrogen atom.