Thanks @jmisiewicz . Just to update this thread, I was having some issues with convergence for some dihedral values, even though I was starting with the torsion at the desired angle. In that case, I opted to just use “frozen_dihedral” instead, which resulted in much more stable calculations.
For reference, the modified section looked like:
# Fixing dihedral term
dihedral_string = "2 1 5 6"
set optking {
frozen_dihedral = $dihedral_string
}