Thanks Rollin for the feedback. By setting FIXED_COORD_FORCE_CONSTANT to 160.0 I was able to optimize butane’s constrained 30, 90 and 150 degree angle (CCCC) using the gau_tight convergence criteria. Note that this value was found by iteratively increasing it from 0.5 to 160.0 (e.g. 0.5, 1.0, 1.5, 3.0, 10.0, etc). Consequently, the specification of intrafrag_step_limit and dynamic_level in my example above was not needed.
I can also confirm that the resulting relative energy profile and geometries are in very good agreement with the results that Gamess provides. (Small note: the Gamess calculations were done without using DF.)
Bests,
Karl