The Foldy–Wouthuysen transform is widely used in high-energy physics. It was historically formulated by Leslie Lawrence Foldy and Siegfried Adolf Wouthuysen in 1949 to understand the nonrelativistic limit of the Dirac equation, the equation for spin-1/2 particles (Foldy and Wouthuysen, 1950; Foldy, 1952; see also Pryce, 1948; Tani, 1951; see Acharya and Sudarshan, 1960 for a detailed general discussion of the Foldy–Wouthuysen-type transformations in particle interpretation of relativistic wave equations). The approach of Foldy and Wouthuysen used a canonical transform that has come to be known as the Foldy–Wouthuysen transformation (a brief account of the history of the transformation is found in the obituaries of Foldy and Wouthuysen [Brown et al., 2001; Leopold, 1997] and the biographical memoir of Foldy [2006]). Before their work, there was some difficulty in understanding and gathering all the interaction terms of a