Akm[k_,m_]:=Piecewise[{{(Epxpx + Epypy + Epzpz)/3, k == 0 && m == 0}, {0, k != 2 || (m != -2 && m != -1 && m != 0 && m != 1 && m != 2)}, {(5*(Epxpx + (2*I)*Epxpy - Epypy))/(2*Sqrt[6]), k == 2 && m == -2}, {(5*(Epxpz + I*Epypz))/Sqrt[6], k == 2 && m == -1}, {(-5*(Epxpx + Epypy - 2*Epzpz))/6, k == 2 && m == 0}, {(-5*(Epxpz - I*Epypz))/Sqrt[6], k == 2 && m == 1}}, (5*(Epxpx - (2*I)*Epxpy - Epypy))/(2*Sqrt[6])]