code: @Override
      public DescriptorValue calculate(IAtom atom, IAtomContainer container) {
          double distanceToAtom;
  
          IAtom focus = container.getAtom(focusPosition);
  
          if (atom.getPoint3d() == null || focus.getPoint3d() == null) {
              return new DescriptorValue(getSpecification(), getParameterNames(), getParameters(), new DoubleResult(
                      Double.NaN), getDescriptorNames(), new CDKException(
                      "Target or focus atom must have 3D coordinates."));
  
          }
  
          distanceToAtom = calculateDistanceBetweenTwoAtoms(atom, focus);
  
          return new DescriptorValue(getSpecification(), getParameterNames(), getParameters(), new DoubleResult(
                  distanceToAtom), getDescriptorNames());
  
      }
docs: This method calculate the 3D distance between two atoms.
  
  @param  atom              The IAtom for which the DescriptorValue is requested
  @param  container         Parameter is the atom container.
  @return                   The number of bonds on the shortest path between two atoms