  We have studied the fundamental process of hydrogen binding at interstitial,
vacancy and grain boundary (GB) in palladium crystals using Density-Functional
Theory. It showed that hydrogen prefers to occupy the octahedral interstitial
site in Pd matrix, however a stable H-vacancy complex with most H occupations
would contain up to eight hydrogen atoms surrounding the vacancy at tetrahedral
sites. Furthermore, H presence assists the pairing or formation of nearby
vacancies, which in agreement with previous suggestions by both experiment and
theory investigation. Also, this observation could imply about a hydrogen
embrittlement (HE) mechanism through the connections of microvoid and cracks.
The segregation of hydrogen at grain boundary, nevertheless, has shown a
different effect. High H accumulation results in grain boundary extension,
which is related the HE mechanism of grain decohesion observed by experiments.
