An update to the BSR–RMT interface for time-dependent laser–atom interactions

We recently developed a computer code [1], which established an interface between the BSR (B-spline R-matrix) and RMT (R-matrix with Time Dependence) packages. The structure information from the former can now serve as input to the latter and thereby be employed in strong-field laser-atom physics calculations. The advantage of BSR is the use of compact, term-dependent, non-orthogonal sets of target orbitals. Based on the many successes of BSR to treat structure, charged-particle collisions, and steady-state photoionization [2], the method is expected to perform particularly well for complex, open-shell targets. BSR-RMT has already proven successful in treating single-configuration LS-coupled targets in multiphoton and high-order harmonic generation processes. We report on extensions to this first version, wherein we have (i) extended the capability of BSR-RMT to treat multi-configurational targets within nonrelativistic LS-coupling, and (ii) incorporated semi-relativistic effects through perturbative Breit-Pauli operators, thereby enabling intermediate-coupling (LSJ) and jK-coupled descriptions as well as an effective approximation to jj-coupling for heavier systems.

[1] J. C. Del Valle, et al., Atoms 13 (2025) 75

[2] O. Zatsarinny and K. Bartschat, J. Phys. B 46 (2013) 112001