Relativistic Wigner function and consistent classical limit for spin 1/2   particles
  The relativistic Wigner function for spin 1/2 particles is the subject of active research due to diverse applications. However, further progress is hindered by the fabulous complexity of the integro-differential equations of motion. We simplify these equations to partial differential equations of the Dirac type that are not only well suited for numerical computation, but also posses a well defined classical limit in a manifestly covariant form. 
Renan Cabrera, Denys I. Bondar, Herschel A. Rabitz
quant-ph physics.class-ph