ASTRA: Transition-density-matrix approach to molecular ionization

We describe astra (attosecond transitions), a close-coupling approach to molecular ionization that uses many-body transition-density matrices between ionic states with arbitrary spin and symmetry, in combination with hybrid integrals between Gaussian and numerical orbitals, to efficiently evaluate photoionization observables. Within the transition-density-matrix approach, the evaluation of interchannel coupling is exact and does not depend on the size of the configuration-interaction space of the ions. Thanks to these two crucial features, astra opens the way to studying highly correlated and comparatively large targets at a manageable computational cost. Here, astra is used to predict the parameters of bound and autoionizing states of the boron atom and of the N₂ molecule, as well as the total photoionization cross section of boron, the nitrogen molecule (N₂), and formaldehyde (H₂CO). Our results are in excellent agreement with theoretical and experimental values from the literature.