Hi Ulf,
well, I'm pretty sure the Espresso with current definition of bond angle won't allow creating a zero angle (or full angle, doesn't matter).
You may set phi0 to whatever value, it will always end up with an angle in between pi/2 and pi..
It's a good remark however, that I wrongly defined the shift which should have been introduced as cosine of the value at least.
However, it won't resolve the problem with defining two bond angle potential simultaneously which Espresso seems to dislike at the moment.
Kind regards
Dusan