On the squared spin-orbit correction to the positronium fine-structure splitting
Abstract
In the recent paper by Zhang the order
α
4
R
∞
corrections to the positronium P levels were reconsidered. Those calculations confirm our corresponding results, except for the contribution due to the squared spin-orbit interaction. We present here a new derivation of our previous result for this last correction, this derivation being to our opinion both simple and convincing.