Akm[k_,m_]:=Piecewise[{{(Ea + Ebx2y2 + Ebxy + 2*Ee)/5, k == 0 && m == 0}, {0, (k != 4 && (k != 2 || m != 0)) || (m != -4 && m != 0 && m != 4)}, {Ea - Ebx2y2 - Ebxy + Ee, k == 2 && m == 0}, {(3*Sqrt[7/10]*(Ebx2y2 - Ebxy + (2*I)*Mb))/2, k == 4 && m == -4}, {(3*(6*Ea + Ebx2y2 + Ebxy - 8*Ee))/10, k == 4 && m == 0}}, (3*Sqrt[7/10]*(Ebx2y2 - Ebxy - (2*I)*Mb))/2]