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

Knowledge about the electronic motion in molecules is essential for our understanding of chemical reactions and biological processes. The advent of attosecond techniques opens up the possibility to induce electronic motion, observe it in real time, and potentially steer it. A fundamental question remains the factors influencing electronic decoherence and the role played by nuclear motion in this process. Here, we simulate the dynamics upon ionization of the polyatomic molecules paraxylene and modified bismethylene-adamantane, with a quantum mechanical treatment of both electron and nuclear dynamics using the direct dynamics variational multiconfigurational Gaussian method. Our simulations give new important physical insights about the expected decoherence process. We have shown that the decoherence of electron dynamics happens on the time scale of a few femtoseconds, with the interplay of different mechanisms: the dephasing is responsible for the fast decoherence while the nuclear overlap decay may actually help maintain it and is responsible for small revivals. DOI: 10.1103/PhysRevLett.118.083001 Electronic motion initiates specific rearrangements of atoms in molecules that are responsible for chemical reactions and biological processes. Because of the advent of attosecond techniques [1,2], it is possible to induce electron dynamics in molecules. Observing and potentially steering electronic motion on its natural time scale may provide novel pathways towards controlling chemical processes [3][4][5][6][7][8]. Since the electron distribution is usually considered to be changing much faster than the nuclear geometry, many theoretical studies treat molecular electron dynamics upon ionization as a purely electronic process, at a single static nuclear geometry [9][10][11][12]: long-lived oscillatory charge migration is then predicted. The fixed-nuclei and single-geometry approximations have however limited validity [13][14][15][16][17][18][19]. The fundamental challenge is to understand to what extent the electronic wave packet retains its coherence, i.e., how long the oscillations in the electronic density survive, in the presence of interactions with the nuclear degrees of freedom.Using a semiclassical description for the coupled systembath evolution, Fiete and Heller identified three processes that contribute to decoherence of the quantum system [20]: (i) system wave packet displacement, (ii) bath overlap decay, and (iii) phase jitter. In the context of molecular electron dynamics, the "system" consists of the electrons and the "bath" of the nuclei. The three mechanisms above can respectively be interpreted as (i) change in the electronic state populations, (ii) decrease of the overlap between the nuclear wave packets on different electronic states, and (iii) dephasing of the different wave packet components. The importance of these mechanisms on the coherent electron dynamics upon molecular ionization remains an outstanding question, that we aim to address in the present Letter.Previous works showed that the n...

Photoisomerization involving a conical intersection (CI) for a model protonated Schiff base (PSB) in modeled water and acetonitrile solvents is examined with the inclusion of energy- and momentum-transfer effects described via a generalized Langevin equation (GLE) frictional approach and surface-hopping dynamics. Short-time GLE frictional effects on the model's three coordinates, the intramolecular bond length alternation and torsional PSB coordinates and a solvent coordinate, eliminate several unphysical features associated with a no-friction inertial description and have the general feature of accelerating nonadiabatic transitions to the ground electronic state. The inertial prediction of equal probability formation of ground-state trans and cis isomer products subsequent to the Franck-Condon excitation of the ground cis isomer is replaced by the GLE prediction of a preferential higher proportion of ground-state trans isomer, that is, a successful cis to trans photoreaction. This preference is solvent-dependent and is enhanced in water solvent with its higher friction intensity and short time scales. For the fast water solvent motion, the nonadiabatic transitions to the S(0) ground state are centered around the CI seam (which is due to the solvent coordinate's role as a tuning coordinate), facilitating direct transitions to the ground-state trans isomer. In contrast, for the slower acetonitrile solvent motion, the decay occurs, on average, away from the CI seam in regions with a finite free-energy gap between the excited and ground states, resulting in reduced trans isomer production. Some directions for the extension of the model description are also discussed.

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 wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with post calculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory and new electronic and muonic basis sets.