Energy levels of doubly excited states of He

The doubly excited states of He, having both electrons in the same n shell, are investigated by constructing a diagonal approximation for R₁₂² in terms of SO(4) operators. By inverting this operator and taking the square root, an approximate operator expression for 1R₁₂ is obtained. Energies of states in the (n=3)² shell of He are calculated using this expression and compared with results of more elaborate calculations. Good agreement is found in most cases.