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hi,
can someone comment on how to decide on the H--atomX distance when passivating the bottom layer X atoms in a slab geometry. I am planning to passivate bottom layer Ga atoms and keep the 2 bottom layers fixed during surface calculations.
Thanks.

You can perform a separate calculation with a thick slab, and passivate the dangling bonds with the hydrogen-like atoms. After structural optimization, you can use the optimized bond lengths between H and the slab in other calculations.