with e x = (1, 0, 0), e y = (0, 1, 0) e z = (0, 0, 1). ψ n/m are occupied KS-DFT orbitals under periodic boundary conditions (PBCs). The Berry phase approach yields the total electric dipole moment for the whole system. Since exp{iG α • r} in Equation( ) is a periodic function, the Berry phase dipole moment is only defined modulo the lattice vector. During an MD simulation, µ Berry α can "jump", which needs to be addressed by adding or subtracting multiples of a α + b α + c α to or from the α component of the dipole vector, to ensure that we autocorrelate dipole moments from the same polarization branch.