The B-spline R-matrix (BSR) approach is an alternative formulation of the well-known R-matrix method developed in Belfast under the long-term leadership of Philip Burke. The program computes transition-matrix elements for electron collisions with many-electron atoms and ions as well as photoionization processes at varying levels of sophistication. From these, cross sections and other experimentally observable quantities can be obtained. Atomic structure information can also be generated with BSR through energy levels and oscillator strengths. The BSR method and the accompanying computer code were developed by Oleg Zatsarinny in the Bartschat group at Drake University.
The published BSR code is a serial version, written using the non-relativistic and semi-relativistic (Breit-Pauli) frameworks. Improvements over the original incarnation of the R-matrix method include: 1) a B-spline basis for the radial functions, considerably enhancing the range and accuracy of the original R-matrix method, and 2) a set of non-orthogonal bound and continuum orbitals enabling high levels of accuracy to be achieved with compact configuration expansions. These improvements allow BSR to describe truly complex systems, e.g., electron scattering from heavy noble gases. The BSR code has also been adapted to study time-dependent electron dynamics such as attosecond photoionization delays in neon and extended to treat fully-relativistic (Dirac) coupling schemes.