Conclusions

In conclusion, the calculations show that Pt^-_s is unstable against a slight displacement along the $\mathopen{<}100\mathclose{>}$ direction. Au_s⁰ shows a similar structure, with a weaker JT distortion. For the TM-H₁ defects, the AB_out configurations were energetically favoured but the differences in energies with the AB_in configurations are not large. We tentatively assign the observed AuH₁ defect to the AB _C1h^out structure. As in previous theoretical studies [#!jones-95!#], we have confirmed the AB structure as the most stable TM-H₂ defect structure, in which the two H atoms lie outside the vacancy.