Hi, 

It is not clear from what you have written - to completely grasp what the problem is. Could you please add some more information?

What is the system you are studying - values will depend on that.

When you say you wrote some code - does that mean you wrote it yourself - or put together libraries? What language are you using?

What exactly is confusing you? Are the values not what you are expecting?

If you add the above information you will be better able to get a coherent answer.

Best,

/A 

@Ashar malik ,

As I said before that I have coded routine program for ewald sum calculation. I know some of the parameters in ewald sum depends on the system configuration like alpha value. The one we provide in lammps and NAMD package for MD . Validation was done with the NIST provided data. Do you know NIST ?  NIST have provided energy value for the sets of 100, 300 and 700 water molecules configuration. They have provided this value in order to validate the self written code. Look at the provided link below. My data is matching well with the NIST. All the energy contribution like dispersion, reciprocal, real , self part are in agreement with the provided value. But they have not calculated the change in energy. In order to calculate the change in energy, we have one simple algorithm but more computational cost. Do you know how to calculate structure factor ? have you heard about structure factor ?

http://www.nist.gov/mml/csd/informatics_research/spce_refcalcs.cfm