Akm[k_,m_]:=Piecewise[{{(Ea1g + 2*(Ee1g + Ee2g))/5, k == 0 && m == 0}, {Ea1g + Ee1g - 2*Ee2g, k == 2 && m == 0}, {(3*(3*Ea1g - 4*Ee1g + Ee2g))/5, k == 4 && m == 0}}, 0]