Quantum chemistry and its “ages”

The popular methods of computational quantum chemistry (CQC) have acquired the status of “mainstream” quantum chemistry (QC), with countless useful applications to molecular science, especially for properties, potential energy surfaces, and reactions of the ground states. In earlier publications, four “ages of QC” have been defined, the classification having been based exclusively on the progress of QC in the direction of CQC.The core of the present article has a dual character: On the one hand, it is a commentary on the nature of QC, whereby it is argued that modern QC, while keeping its focus on the many‐electron problem, (MEP), and on the consequences of Electron Correlations on observable quantities, has a domain and a scope that are larger than those determined, more or less, by CQC. Hence, additional “ages” are singled out and proposed, which are connected to “time‐independent” and “time‐dependent” theoretical “many‐electron” formulations and calculations in which the “continuous spectrum” is involved explicitly. They are connected to experimental directions, which challenge theory to provide not only phenomenology but also quantitative answers and predictions for properties