New Corrections of Order $\alpha^6$ to $s$-Levels of Two-Body Systems
  New corrections to the energy of $S$-levels of positronium of order $m\alpha^6$ which are as large as several hundred kilohertz are obtained. A new recoil correction of order $\alpha(Z\alpha)^5(m/M)m$ to the Lamb shift in hydrogen is calculated. This correction turns out to be too small from the phenomenological point of view. 
Michael I. Eides and Howard Grotch