UKRmol+: A suite for modelling electronic processes in molecules interacting with electrons, positrons and photons using the R-matrix method

UKRmol+ is a new implementation of the time-independent UK R-matrix electron-molecule scattering code. Key features of the implementation are the use of quantum chemistry codes such as Molpro to provide target molecular orbitals; the optional use of mixed Gaussian - B-spline basis functions to represent the continuum and improved configuration and Hamiltonian generation. The code is described, and examples covering electron collisions from a range of targets, positron collisions and photoionization are presented. The codes are freely available as a tarball from Zenodo.

Program Files doi:http://dx.doi.org/10.17632/k3ny7zcfrb.1

Code Ocean Capsule:https://doi.org/10.24433/CO.2477858.v1

Licensing provisions: GNU GPLv3

Programming language: Fortran 95 with use of some Fortran 2003 features

External routines/libraries: LAPACK, BLAS; optionally MPI, ScaLAPACK, Arpack, SLEPc

Nature of problem: The computational study of electron and positron scattering from a molecule requires the determination of multicentric time-independent wavefunctions describing the target+projectile system. These wavefunctions can also be used to calculate photoionization cross sections (in this case the free particle is the ionized electron) or provide input for time-dependent calculations of laser-induced ultrafast processes.

Solution method: We use the R-matrix method [1], that partitions space into an 'inner' and an 'outer' region. In the inner region (within a few tens of a₀ of the nuclei at most) exchange and correlation are taken into account. In the outer region, where the free particle is distinguishable from the target electrons, a single-centre multipole potential describes its interaction with the molecule. The key computational step is the building and diagonalization of the target + free particle Hamiltonian in the inner region, making use of integrals generated using the GBTOlib library. The eigenpairs obtained are then used as input to the outer region suite to determine scattering quantities (K-matrices, etc.) or transition dipole moments and, from them, photoionization cross sections. The suite also generates input data for the R-matrix with time (RMT) suite [2].

Additional comments: CMake scripts for the configuration, compilation, testing and installation of the suite are provided. This article describes the release version UKRmol-in 3.0, that uses GBTOlib 2.0, and UKRmol-out 3.0.

Program repository available at: https://gitlab.com/UK-AMOR/UKRmolReferences

[1] P. G. Burke, R-Matrix Theory of Atomic Collisions: Application to Atomic, Molecular and Optical Processes. Springer, 2011.

[2] A. Brown, et al RMT: R-matrix with time-dependence. Solving the semi-relativistic, time-dependent Schrödinger equation for general, multi-electron atoms and molecules in intense, ultrashort, arbitrarily polarized laser pulses., Computer Phys. Comm., https://doi.org/10.1016/j.cpc.2019.107062.