Here, δ δ ( x ) is the Dirac delta function, a 1 , a 2 and a 3 are the lattice vectors and n 1 , n 2 and n 3 range over all integers. The total field ρ ρ TOT ( r ) can be represented as a convolution of ρ ρ u c ( r ) with a lattice function L ( r ) L ( r ) = d e f ∑ ∑ n 1 , n 2 , n 3 δ δ ( r − − n 1 a 1 − − n 2 a 2 − − n 3 a 3 )