This shows how the effective momentum can be seen as composed of two parts, p ^ ^ eff = − − i ℏ ℏ ∇ ∇ + ℏ ℏ k , a standard momentum − − i ℏ ℏ ∇ ∇ and a crystal momentum ℏ ℏ k . More precisely the crystal momentum is not a momentum but it stands for the momentum in the same way as the electromagnetic momentum in the minimal coupling, and as part of a canonical transformation of the momentum.