Akm[k_,m_]:=Piecewise[{{(Ea + Ebx + Eby)/3, k == 0 && m == 0}, {0, k != 2 || (m != -2 && m != 0 && m != 2)}, {(5*(Ebx - Eby + (2*I)*Mb))/(2*Sqrt[6]), k == 2 && m == -2}, {(5*(2*Ea - Ebx - Eby))/6, k == 2 && m == 0}}, (5*(Ebx - Eby - (2*I)*Mb))/(2*Sqrt[6])]