Some important points:
1. Make sure all atoms are present (hydrogen atoms or solvent molcules are sometimes missing).
2. Choose your energy potential. How accurate do your energies have to be? A good method is dispersion-corrected density functional theory, but you'd need access to the right software and hardware to do such a calculation. Note that these energies will not be free energies but energies/enthalpies.
3. Energy minimise your structure with the energy potential, with the unit cell free.
4. What is "lattice energy"? For a single structure, "the energy" is meaningless, you can only calculate energy differences, e.g. between two polymorphs, but note that energy potentials will not be accurate enough to give the correct stability order at room temperature. You can calculate the energy of the entire crystal structure and subtract the energy of one of the molecules to get the packing energy (assuming you have a molecular compound).
You can start by talking to your local crystallographer who may have some pointers. If you want publications, search for Gavezzotti, Price, Neumann, Day.
Jacco's answer is great. I can perhaps add that the program Quantum ESPRESSO is freely available and can do density functional calculations of this kind.
You need to choose which functional to use. I would recommend PBE with some dispersion-correction, it is by far the most commonly used method. See papers by Reilly, and Tkatchenko for benchmarks.
Once you have calculated the energy of the crystal, you need to extract the coordinates of a single molecule and do the calculation on it. Use a periodic box of say 20 Angstrom with the single molecule in it. Then you can calculate the lattice energy from the crystal's energy and the molecule's.
There are also fairly accurate force field methods, see papers by Price and Nyman & Day in PCCP 2016, but the programs used for such force fields with multipoles are hard to use. If it's only one or two structures, you could ask one of us to do it for you.
Thanks a lot Mr. Jonas Nyman.First of all, let me know the facility present here.After that I shall contact you.Can you give your mail id to which i can contact you?
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