CCL: freeze interval vibrations

["Eric Hu" <list.eric/./gmail.com>]

 Hi, I am conducting an organic molecular assembly simulation (about 55
 atoms in each subunit and there are 12 subunit in total). I want to
 freeze the
 
subunit internal vibration (saving sampling time) during md since
 I am more interested in the assembly process between subunits. 
 
 Are there any suitable "rigid body" molecular dynamics packages for
 this purpose? Thank you.
 
 Eric