Norm conservation of a semidiscrete Schrödinger equation ($i{U}^{\prime }=HU,H=H^{\ast }⟹\Vert U(t)\Vert =\Vert U(0)\Vert$)

If a spatial method of lines has a Hermitian matrix $H$, its generator $-iH$ is a skew-Hermitian matrix. The matrix exponential $e^{-itH}$ is a unitary matrix, so it preserves the Euclidean norm and any constant-volume discrete L2 norm. Equivalently, differentiating the squared norm gives $2\mathrm{Re}(-i{U}^{\ast }HU)=0$. A real sampled potential preserves the Hermitian property of a symmetric discrete Laplacian. Subsequent time discretization must be assessed separately.