Dirac R-matrix with pseudo-states

We describe the development of the Dirac R-matrix with pseudo-states (DRMPS) method for electron and photon collisions with arbitrary atoms and ions. An N-electron atom is represented by an anti-symmetrized product of single-particle spinors comprising the usual four-component Dirac spinors as well as paired two-component Laguerre spinors (L-spinors). The convergent L-spinor basis forms a discretization of the electron and positron continua. This representation has been implemented quite generally within the general relativistic atomic structure package (GRASP) specifically GRASP(0). An (N + 1)th 'scattering' electron is represented by the exact same L-spinor basis plus the usual R-matrix box-state spinors which are chosen so as to form a combined complete finite linearly independent orthogonal basis. The (non-diagonal) Buttle correction is determined consistently. This representation has been implemented quite generally within the Dirac atomic R-matrix code (DARC). Results of some simple model problems are presented which demonstrate the correctness of the implementation.