ASTRA: Transition-density-matrix approach to molecular ionization

We describe ASTRA (attosecond transitions), a close-coupling approach to molecular ionization that uses manybody 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.