Akm[k_,m_]:=Piecewise[{{(Ea1g + 2*(Eeg\[Pi] + Eeg\[Sigma]))/5, k == 0 && m == 0}, {Ea1g - Eeg\[Pi] - 2*Sqrt[2]*Meg, k == 2 && m == 0}, {(1/2 - I/2)*Sqrt[7/5]*(2*Eeg\[Pi] - 2*Eeg\[Sigma] - Sqrt[2]*Meg), k == 4 && m == -3}, {(9*Ea1g - 2*Eeg\[Pi] - 7*Eeg\[Sigma] + 10*Sqrt[2]*Meg)/5, k == 4 && m == 0}, {(-1/2 - I/2)*Sqrt[7/5]*(2*Eeg\[Pi] - 2*Eeg\[Sigma] - Sqrt[2]*Meg), k == 4 && m == 3}}, 0]