The wave functions are the solution of a field-theoretic analog of the Schrödinger equation H ψ ψ = E ψ ψ of nonrelativistic quantum mechanics. In the nonrelativistic theory the Hamiltonian operator H is just a kinetic piece − − ℏ ℏ 2 2 m ∇ ∇ 2 and a potential piece V ( r → → ) . The wave function ψ ψ is a function of the coordinate r → → , and E is the energy. In light-front quantization, the formulation is usually written in terms of light-front momenta p _ _ i = ( p i + , p → → ⊥ ⊥ i ) , with i a particle index, p i + ≡ ≡ p i 2 + m i 2 + p i z , p → → ⊥ ⊥ i = ( p i x , p i y ) , and m i the particle mass, and light-front energies p i − − ≡ ≡ p i 2 + m i 2 − − p i z . They satisfy the mass-shell condition m i 2 = p i + p i − − − − p → → ⊥ ⊥ i 2