I=2 $\pi\pi$ scattering using G-parity boundary condition

To make the $\pi\pi$ state with non-zero relative momentum as the leading exponential, we impose anti-periodic boundary condition on the pion, which is implemented by imposing G-parity or H-parity on the quark fields at the boundary. With this, we calculate the I=2 $\pi\pi$ phase shift from lattice simulation by using L\"uscher's formula.