FFEA allows to add different types of restraints, through springs, and pinned nodes and frozen blobs. The following sections go through the detail on what is computed in every case.

Springs

Adding distance restraints between nodes belonging to the same or to different Blobs is done in FFEA using springs between the nodes. In that case, the user is able to specify the strength and equilibrium distance of the Hookean potential. In order to use restraints, one has set:

< calc_springs = 1 >

in the input .ffea file. In addition to that, <system> has to include the <interactions> block with the <springs> block, where the springs_fname is specified. Thus, a valid input ffea file simulating two blobs with restraints would have a structure:

<param>
   ...
   <calc_springs=1>
   ...
</param>
<system>
   <blob>
      ...
   </blob>
   <blob>
      ...
   </blob>
   <interactions>
      <springs>
         <springs_fname=myspringsfile.springs>
      </springs>
   </interactions> 
</system>

It is mandatory that the springs_fname suffix is "springs". Full description of the format of the input springs_fname can be found here.

Freezing degrees of freedom

Degrees of freedom can be individually or collectively removed through pinned nodes or freezing blobs. In any case, the existing active faces within the frozen structures as well as those beads assigned to the corresponding elements will remain involved in the calculation of forces that will be applied to the rest of the structures.

Pinned nodes

If using a list of pinned nodes through pin one has to pass in a .pin file with a list of nodes. The position of the nodes within that list will never change during simulation.

Frozen blob

One can freeze a whole blob passing the flag STATIC or FROZEN to the motion_state of a Blob conformation. Again, the position of any of the nodes within this conformation will never be updated.