The bound current is derived from the P and M fields which form an antisymmetric contravariant magnetization-polarization tensor M μ μ ν ν = ( 0 P x c P y c P z c − − P x c 0 − − M z M y − − P y c M z 0 − − M x − − P z c − − M y M x 0 ) , which determines the bound current J ν ν bound = ∂ ∂ μ μ M μ μ ν ν .