to be used for computations of large systems. In addition, the report includes the description of a computational machinery for nonlinear optical spectroscopy through an interface to the QM/MM package Cobramm. Further, a module to run molecular dynamics simulations is added and two surface hopping algorithms are included to enable nonadiabatic calculations. Finally, we report on the subject of improvements with respects to alternative file options and parallelization.

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

MOLCAS/OpenMolcas is an ab initio electronic structure program providing a large set of computational methods from Hartree–Fock and density functional theory to various implementations of multiconfigurational theory. This article provides a comprehensive overview of the main features of the code, specifically reviewing the use of the code in previously reported chemical applications as well as more recent applications including the calculation of magnetic properties from optimized density matrix renormalization group wave functions.

Abstract:The valence excited states of ferric and ferrous hexacyanide ions in aqueous solution were mapped with resonant inelastic X-ray scattering (RIXS) at the Fe L2,3-and N K-edges. Probing of both the central Fe and the ligand N atoms enabled identification of the metal-and ligand-centered excited states, as well as ligandto-metal and metal-to-ligand charge transfer excited states. Ab initio calculations utilizing the RASPT2 method was used to simulate the Fe L2,3-edge RIXS spectra and enabled quantification of the covalency of both occupied and empty orbitals of π and σ symmetry. We find that π back-donation in the ferric complex is smaller compared to the ferrous complex. This is evidenced by the relative amount of Fe 3d character in the nominally 2π CN -molecular orbital of 7% and 9% in ferric and ferrous hexacyanide, respectively. Utilizing the direct sensitivity of Fe L3-edge RIXS to the Fe 3d character in the occupied molecular orbitals we also find that the donation interactions are dominated by σ-bonding. The latter is found to be stronger in the ferric complex with a Fe 3d contribution to the nominally 5σ CN -molecular orbitals of 29% compared to 20% in the ferrous complex. These results are consistent with the notion that a higher charge at the central metal atom increases donation and decreases back-donation.

PostprintThis is the accepted version of a paper published in Journal of Chemical Physics. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the original published paper (version of record):Pinjari, R V., Delcey, M G., Guo, M., Odelius, M., Lundberg, M. (2014) Restricted active space calculations of L-edge X-ray absorption spectra: From molecular orbitals to multiplet states. The metal L-edge (2p → 3d) X-ray absorption spectra are affected by a number of different interactions; electron-electron repulsion, spin-orbit coupling, and charge transfer between metal and ligands, which makes the simulation of spectra challenging. The core restricted active space (RAS) method is an accurate and flexible approach that can be used to calculate X-ray spectra of a wide range of medium-sized systems without any symmetry constraints. Here, the applicability of the method is tested in detail by simulating three ferric (3d 5 ) model systems with well-known electronic structure viz. atomic Fe 3+ , high-spin [FeCl 6 ] 3-with ligand donor bonding, and low-spin [Fe(CN) 6 ] 3-that also has metal backbonding. For these systems, the performance of the core RAS method, which does not require any system-dependent parameters, is comparable to that of the commonly used semi-empirical charge-transfer multiplet model. It handles orbitally degenerate ground states, accurately describes metal-ligand interactions, and includes both single and multiple excitations. The results are sensitive to the choice of orbitals in the active space and this sensitivity can be used to assign spectral features. A method has also been developed to analyze the calculated X-ray spectra using a chemically intuitive molecular orbital picture. Journal of Chemical

PostprintThis is the accepted version of a paper published in Journal of Computational Chemistry. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination. Citation for the original published paper (version of record):Pinjari, R V., Delcey, M G., Guo, M., Odelius, M., Lundberg, M. (2016) Cost and sensitivity of restricted active-space calculations of metal L-edge X-ray absorption spectra. Abstract The restricted active space (RAS) approach can accurately simulate metal L-edge X-ray absorption spectra of first-row transition metal complexes without the use of any fitting parameters. These characteristics provide a unique capability to identify unknown chemical species and to analyze their electronic structure. To find the best balance between cost and accuracy, the sensitivity of the simulated spectra with respect to the method variables have been tested for two models, [FeCl 6 ] 3-and [Fe(CN) 6 ] 3-. For these systems the reference calculations give deviations, compared to experiment, of 1 eV in peak positions, 30% for the relative intensity of major peaks, and 50% for minor peaks. Compared to these deviations, the simulated spectra are sensitive to the number of final states, the inclusion of dynamical correlation and the ionization potential-electron affinity (IPEA) shift, in addition to the selection of the active space. The spectra are less sensitive to the quality of the basis set and even a double-ζ basis gives reasonable results. Inclusion of dynamical correlation through second-order perturbation theory can be done efficiently using the state-specific formalism without correlating the core orbitals. Although these observations are not directly transferable to other systems, they can, together with a cost analysis, aid in the design of RAS models and help extend the use of this powerful approach to a wider range of transitionmetal systems. Journal of Computational Keywords:transition metals, X-ray absorption spectroscopy, multiconfigurational wavefunction, spin-orbit coupling, charge transfer TABLE OF CONTENTSWith an appropriate choice of active space, basis set and computational procedure the restricted active space approach can be used to simulate metal L-edge X-ray absorption spectra with reasonable accuracy and computational cost. The sensitivity of the simulated results with respect to geometrical changes opens up for analysis of dynamical processes.2

state-average complete-active-space SCF derivative (nonadiabatic) coupling vectors are implemented. Existing formulations are modified such that the implementation is compatible with Cholesky-based density fitting of twoelectron integrals, which results in efficient calculations especially with large basis sets. Using analytical nonadiabatic coupling vectors, the optimization of conical intersections is implemented within the projected constrained optimization method. The standard description and characterization of conical intersections is reviewed and clarified, and a practical and unambiguous system for their classification and interpretation is put forward. These new tools are subsequently tested and benchmarked for 19 different conical intersections. The accuracy of the derivative coupling vectors is validated and the information that can be drawn from the proposed characterization is discussed, demonstrating its usefulness. even if one is willing to allow for numerical differentiation. To overcome this limitation, several algorithms have been proposed that obviate the need for the full derivative coupling vector. For example, the fewest-switches surface hopping algorithm for nonadiabatic molecular dynamics 13 requires only the dot product of the derivative coupling and the velocity vector, which is straightforward to evaluate from the wavefunctions at different timesteps. For the optimization of conical intersections there are also algorithms that do not rely on a derivative coupling vector. 14-16 Nevertheless, analytical formulations have been published for several electronic structure methods, 17-22 and using analytical derivative couplings is almost always preferable to ad hoc numerical differentiation or approximate methods.Molcas 23 is one of the leading software packages when it comes to multiconfigurational methods, due to the efficiency and versatility it offers. However, until now Molcas lacked the ability to compute derivative couplings, analytically or numerically, which limited its potential for use in state-of-the-art applications in the field of nonadiabatic processes. In this work, we address this problem by implementing analytical derivative coupling vectors for state-average complete active space self-consistent field (SA-CASSCF) wavefunctions, the cornerstone of most multiconfigurational calculations with Molcas. Although analytical formulations for general multiconfigurational self-consistent field (MCSCF) and multireference configuration interaction (MRCI) derivative coupling vectors have been published before, 17,19 we simplify the expressions for the specific case of SA-CASSCF and connect them with the formulation for SA-CASSCF energy gradients already used in Molcas. 24 In the last few years, the performance of Molcas has been greatly improved with the implementation of a Cholesky decomposition scheme for two-electron integrals, its reformulation as a particular form of density fitting, and the exploitation of this in most parts of the code, including analytical derivatives. [25...

The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.