Method ofKharmonics and the ground-state energy of the positronium negative ion

The method of $K$ harmonics, which has been successfully used to find some of the basic characteristics of a nucleus, is applied to determine the ground-state energy of the positronium negative ion, $e^{-}{e}^{+}{e}^{-}$. Retaining $K$ harmonics up to $K=2\mathrm{}$ in the expansion for the wave function, the ground-state energy comes out as -0.256 (in a.u.). The energy value improves to -0.259 when the $K$ harmonics up to $K=4\mathrm{}$ are included.