Akm[k_,m_]:=Piecewise[{{(2*Eeg + 3*Et2g)/5, k == 0 && m == 0}, {(1 - I)*Sqrt[7/5]*(Eeg - Et2g), k == 4 && m == -3}, {(-7*(Eeg - Et2g))/5, k == 4 && m == 0}, {(-1 - I)*Sqrt[7/5]*(Eeg - Et2g), k == 4 && m == 3}}, 0]