Dear Naveen, In a square lattice for instance \exp(2 i\pm k.r) would rather correspond to a third nearest neighbor. This may be the reason of the discrepancy you observe.
Regards, Ousmane
Dear Naveen, In a square lattice for instance \exp(2 i\pm k.r) would rather correspond to a third nearest neighbor. This may be the reason of the discrepancy you observe.
Regards, Ousmane