  Hydrogen embrittlement of metals is widely observed, but its atomistic
origins remain little understood and much debated. Combining a unique
identification of interstitial sites through polyhedral tessellation and
first-principles calculations, we study hydrogen adsorption at grain boundaries
in a variety of face-centered cubic metals of Ni, Cu, gamma-Fe and Pd. We
discover the chemomechanical origin of variation of adsorption energetics for
interstitial hydrogen at grain boundaries. A general chemomechanical formula is
established to provide accurate assessments of hydrogen trapping and
segregation energetics at grain boundaries, and it also offers direct
explanations for certain experimental observations. The present study deepens
our mechanistic understanding of the role of grain boundaries in hydrogen
embrittlement, and promises a viable path towards predictive microstructure
engineering against hydrogen embrittlement in structural metals.
