Akm[k_,m_]:=Piecewise[{{(Ea1g + Eb1g + Eb2g + 2*Eeg)/5, k == 0 && m == 0}, {Ea1g - Eb1g - Eb2g + Eeg, k == 2 && m == 0}, {(3*Sqrt[7/10]*(Eb1g - Eb2g))/2, k == 4 && (m == -4 || m == 4)}, {(3*(6*Ea1g + Eb1g + Eb2g - 8*Eeg))/10, k == 4 && m == 0}}, 0]