Gulp optimisation and MD can produce odd H atom positions when starting from high energy structures.

[andrewtarzia]

I have found many examples of structures after UFF4MOF optimisation or high T MD (using Gulp through stko), where H atoms near binding sites will be pointing the wrong way relative to their C atom. This issue normally resolves itself with subsequent GFN2-xTB optimisation or with higher T MD (not always the case for the MD, which suggests this is a local minima for the H atom).

Unsure the exact cause, but considering the initial "high energy" nature of stk molecules, with long bonds and bad inter-building block angles (even when using MCHammer of Collapser), this may not be so surprising and users should be careful.

As I do not know the cause, I assume it is due to high energy initial structures, I also do not know the fix from an stko perspective. Maybe it is the way the force field is described or the effect of typing (a Gulp issue).