where c is the speed of light, E is the nonrelativistic particle energy, p = − − i ℏ ℏ ∇ ∇ is the momentum operator, and σ σ = ( σ σ x , σ σ y , σ σ z ) is the vector of Pauli matrices, which is proportional to the spin operator S = 1 2 ℏ ℏ σ σ . Here ψ ψ , χ χ are two components functions (spinors) describing the wave function of the particle.