Linear and circular constant forces can be added onto nodes and surfaces, selected either individually or collectively. In order to do that, one needs to activate the calculation of the constant forces:

< calc_ctforces = 1 >

in the input .ffea file. In addition to that, <system> has to include the <interactions> block with the <ctforces> block, where the ctforces_fname needs to be specified. Thus, a valid input ffea file simulating two blobs with constant forces would have the structure:

<param>
   ...
   <calc_ctforces=1>
   ...
</param>
<system>
   <blob>
      ...
   </blob>
   <blob>
      ...
   </blob>
   <interactions>
      <ctforces>
         <ctforces_fname=myspringsfile.ctforces>
      </ctforces>
   </interactions>
</system>

Full description of the format of the input ctforces_fname file can be found here

FFEA