Linear response to external Em field Perturbed hamiltonian H=∑v( D÷A+重3)亚1 pq ∑ p9 P+q pt q, p)Aop Perturbed current 6,J +or-7)A( Response function (r{y“(r,q)y(r,-q)) ∑(T-tn(r)"p+q:p)p+4)oty+4"(p,p+gyvp( But it cannot satisfy current conservationLinear response to external EM field Perturbed Hamiltonian Perturbed current Response function 0 = 0  But it cannot satisfy current conservation qQ