  Palladium is an ideal system for understanding the behavior of hydrogen in
metals. In Pd, H is located both in octahedral sites and in dislocation cores,
which act as nanoscale H traps and form Cottrell atmospheres. Adjacent to a
dislocation core, H experiences the largest possible distortion in alpha-Pd. Ab
initio density-functional theory computes the potential energy for a hydrogen
in an octahedral site in alpha-Pd and in a trap site at the core of a partial
of an edge dislocation. The Pd partial dislocation core changes the environment
for H, distorting the H-Pd bonding which changes the local potential,
vibrational spectra, and inelastic form factor for an isolated H atom. The
decrease in excitation energy is consistent with experiments, and the
calculations predict distortions to the H wavefunction.
