Updated doc page with delta info and simplified example

.. 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 LAMMPS

unit style, 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::

The cost of each energy evaluation is essentially the cost of an MD

timestep.  This invoking this fix once has a cost of 2N timesteps,

where N is the total number of atoms in the system (assuming all atoms

#############################

#Atoms types - mass - charge#

#############################

#@ 1 atom types #!THIS LINE IS NECESSARY DON'T SPEND HOURS FINDING THAT OUT!#

variable Ar equal 1

#############

#Atom Masses#

#############

mass ${Ar}      39.903

###########################

#Pair Potentials - Tersoff#

###########################

pair_style      lj/cubic

pair_coeff      * * 0.0102701 3.42

# Numerical difference calculation of error in forces

units   	metal

atom_style      full

atom_style      atomic

boundary 	p p p

read_data argon.lmp

include ff-argon.lmp

lattice fcc 5.358000

region box block 0 6 0 6 0 6

create_box 1 box

create_atoms 1box

mass 1 39.903

velocity all create 10 2357 mom yes dist gaussian

pair_style lj/cubic

pair_coeff * * 0.0102701 3.42

neighbor 1 bin

timestep 0.001