Gauge invariance of many-body Schrödinger equation with explicit Coulomb potential

Abstract: A simple argument is presented which, based on the minimal coupling Lagrangian for a many-body system, keeps the gauge invariance of the many-body Schr\"odinger equation with explicit Coulomb potential. The elimination of longitudinal electric field does not necessarily lead to the breakdown of gauge invariance. The total time derivative term in the matter-EM field interaction in the Lagrangian is canceled out by the choice of Coulomb gauge. The remaining interaction is described by transverse vector potential and the longitudinal electric field which is the homogeneous solution of Gauss law. This leads directly to the gauge invariant forms of the linear and nonlinear constitutive equations. It is discussed how to reconcile this result with the cases of an isolated matter interacting with external charges.