In this article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational

We show that the use of the IPEA correction in CASPT2 for excited state calculations of organic chromophores is not justified.

Nitronaphthalene derivatives efficiently populate their electronically excited triplet states upon photoexcitation through ultrafast intersystem crossing (ISC). Despite having been studied extensively by time‐resolved spectroscopy, the reasons behind their ultrafast ISC remain unknown. Herein, we present the first ab initio nonadiabatic molecular dynamics study of a nitronaphthalene derivative, 2‐nitronaphthalene, including singlet and triplet states. We find that there are two distinct ISC reaction pathways involving different electronic states at distinct nuclear configurations. The high ISC efficiency is explained by the very small electronic and nuclear alterations that the chromophore needs to undergo during the singlet–triplet transition in the dominating ISC pathway after initial dynamics in the singlet manifold. The insights gained in this work are expected to shed new light on the photochemistry of other nitro polycyclic aromatic hydrocarbons that exhibit ultrafast intersystem crossing.

Absolute differential cross sections for the electronic excitation of valence states in CO have been measured in the energy range from threshold to 3.7 eV. In particular, a newly developed experimental technique to control the analyser's efficiency allowed us to measure absolute angular dependences of those states for scattering angles between and . The transitions from the ground state to the and states are forbidden by spin selection rules. It is found that the shape resonance located at 1.8 eV partly decays into these valence states, which are located several eV above the resonance. The decay occurs by ejection of a core electron whose spin is antiparallel to that of the incident electron causing a sharp rise of the cross sections just at threshold. Three new core-excited shape resonances were found, one for each of , and symmetry. By comparison with recently calculated cross sections their electron configurations have been classified. Resonant excitation via these three resonances or via the low-lying resonance dominates the cross sections of the two spin forbidden transitions in the detected energy range. The excitation of the optically allowed transition, however, is dominated by non-resonant processes.

The developments of the open-source chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes can address, while showing that is an attractive platform for state-of-the-art atomistic computer simulations.