Fixed up delta note

   It is important to choose a suitable value for delta, the magnitude of

   atom displacements that are used to generate finite difference

   approximations to the exact forces.  For typical systems, a value in

   the range 1e-xxx to 1e-yyy will probably work well.  However, the

   best value will depend on a multitude of factors including 

   the stiffness of the interatomic potential,the

   thermodynamic state of the material being probed, and so on. The only

   way to be sure that you have made a good choice is to do a

   sensitivity study on a representative atomic configuration, sweeping 

   over a wide range of values of delta.  If delta is too small, the

   output forces will vary erratically due to truncation effects. If

   delta is increased beyond a certain point, the output forces will

   start to vary smoothly with delta, due to growing contributions from

   higher order derivatives. In between these two limits, the numerical

   force values should be largely independent of delta.

   the range of 1 part in 1e4 to 1e5 of the typical separation distance

   between atoms in the liquid or solid state will be sufficient.

   However, the best value will depend on a multitude of factors

   including the stiffness of the interatomic potential, the thermodynamic

   state of the material being probed, and so on. The only way to be sure

   that you have made a good choice is to do a sensitivity study on a

   representative atomic configuration, sweeping over a wide range of

   values of delta.  If delta is too small, the output forces will vary

   erratically due to truncation effects. If delta is increased beyond a

   certain point, the output forces will start to vary smoothly with

   delta, due to growing contributions from higher order derivatives. In

   between these two limits, the numerical force values should be largely

   independent of delta.

.. note::