code: private void setForceField(String ffname, IChemObjectBuilder builder) throws CDKException {
          if (ffname == null) {
              ffname = "mm2";
          }
          try {
              forceFieldName = ffname;
              ffc.setForceFieldConfigurator(ffname, builder);
              parameterSet = ffc.getParameterSet();
          } catch (CDKException ex1) {
              logger.error("Problem with ForceField configuration due to>" + ex1.getMessage());
              logger.debug(ex1);
              throw new CDKException("Problem with ForceField configuration due to>" + ex1.getMessage(), ex1);
          }
      }
docs: Sets the forceField attribute of the ModelBuilder3D object.
  
  @param  ffname  forceField name