I have been trying to use shake to constrain the bond distances of my target molecule, however this does not prevent the bond angles from changing. When I try shake angle then all the atoms in the molecule move wildly.

It is baffling me that I am unable to find a function in CHARMM to perform a task that is not uncommon (preventing any changes to a molecule's shape).

Am I looking at the wrong commands? I have tried:

cons dihe
cons harm abso
cons IC

shake bond
shake angl