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.

Benzene exhibits a rich photochemistry which can provide access to complex molecular scaffolds that are difficult to access with reactions in the electronic ground state. While benzene is aromatic in its ground state, it is antiaromatic in its lowest ππ* excited states. Herein, we clarify to what extent relief of excited-state antiaromaticity (ESAA) triggers a fundamental benzene photoreaction: the photoinitiated nucleophilic addition of solvent to benzene in acidic media leading to substituted bicyclo[3.1.0]hex-2-enes. The reaction scope was probed experimentally, and it was found that silyl-substituted benzenes provide the most rapid access to bicyclo[3.1.0]hexene derivatives, formed as single isomers with three stereogenic centers in yields up to 75% in one step. Two major mechanism hypotheses, both involving ESAA relief, were explored through quantum chemical calculations and experiments. The first mechanism involves protonation of excited-state benzene and subsequent rearrangement to bicyclo[3.1.0]hexenium cation, trapped by a nucleophile, while the second involves photorearrangement of benzene to benzvalene followed by protonation and nucleophilic addition. Our studies reveal that the second mechanism is operative. We also clarify that similar ESAA relief leads to puckering of S 1 -state silabenzene and pyridinium ion, where the photorearrangement of the latter is of established synthetic utility. Finally, we identified causes for the limitations of the reaction, information that should be valuable in explorations of similar photoreactions. Taken together, we reveal how the ESAA in benzene and 6π-electron heterocycles trigger photochemical distortions that provide access to complex three-dimensional molecular scaffolds from simple reactants.

We present a comparative study on the influence of the quantum mechanical (QM) method (including basis set) on the evaluation of transition energies, transition densities and dipoles, and excitation energy transfer (EET) electronic couplings for a series of chromophores (and the corresponding pairs) typically found in organic electro-optical devices and photosynthetic systems. On these systems we have applied five different QM levels of description of increasing accuracy (ZINDO, CIS, TD-DFT, CASSCF, and SAC-CI). In addition, we have tested the effects of a surrounding environment (either mimicking a solvent or a protein matrix) on excitation energies, transition dipoles, and electronic couplings through the polarizable continuum model (PCM) description. Overall, the results obtained suggest that the choice of the QM level of theory affects the electronic couplings much less than it affects excitation energies. We conclude that reasonable estimates can be obtained using moderate basis sets and inexpensive methods such as configuration interaction of single excitations or time-dependent density functional theory when appropriately coupled to realistic solvation models such as PCM.

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.

We examine the Stark component of the solute−solvent interaction energy and check the validity of the mean field approximation (MFA) in the theoretical study of liquids and solutions. We considered two types of systems:  methanol, ethanol, and propanol liquids and formaldehyde, acetaldehyde, and acetone in aqueous solution. We found that, independent of the level of calculation (HF, MP2, or MCSCF), the errors introduced by MFA are less than 5% in the interaction energy and less than 1% in the dipoles. We propose an approximate expression for the Stark component that reduces the errors in the interaction energy to below 1.6%.

The nudged elastic band (NEB) method is a successful optimization method for obtaining minimum energy reaction paths if only the initial and final structures are known. However, the original implementation of the method had some limitations, which has meant that there has been considerable interest in proposing alternative NEB formulations, which show improved convergence behavior. In this work, we present two modifications to the standard NEB procedure. The first involves the use of a second-order quasi-Newton optimization technique applied separately to each of the images that form the path. The second consists of the use of an interpolating spline to represent the path. This ensures that the images along the path are evenly spaced and means that the arbitrary spring forces employed in the standard NEB method are no longer necessary. We tested these modifications on a set of small, but relatively complex, chemical systems and found that the computation time was reduced by as much as 90% compared with the standard method.

Almost all chemiluminescent and bioluminescent reactions involve cyclic peroxides. The structure of the peroxide and reaction conditions determine the quantum efficiency of light emission. Oxidizable fluorophores, the so-called activators, react with 1,2-dioxetanones promoting the former to their first singlet excited state. This transformation is inefficient and does not occur with 1,2-dioxetanes; however, they have been used as models for the efficient firefly bioluminescence. In this work, we use the SA-CASSCF/CASPT2 method to investigate the activated chemiexcitation of the parent 1,2-dioxetane and 1,2-dioxetanone. Our findings suggest that ground state decomposition of the peroxide competes efficiently with the chemiexcitation pathway, in agreement with the available experimental data. The formation of non-emissive triplet excited species is proposed to explain the low emission efficiency of the activated decomposition of 1,2-dioxetanone. Chemiexcitation is rationalized considering a peroxide/activator supermolecule undergoing an electron-transfer reaction followed by internal conversion.

Combining ab initio quantum mechanics with a dipole-field model to describe acid dissociation reactions in water: First-principles free energy and entropy calculations Anomalous conformational behavior of poly(ethylene oxide) oligomers in aqueous solutions. A molecular dynamics studyThe average solvent electrostatic potential/molecular dynamics ͑ASEP/MD͒ and the free-energy gradient methods are applied together with the multidimensional geometry optimization of molecules in solution. The systems studied were formamide in aqueous solution and water and methanol in liquid phase. The solute molecules were described through ab initio quantum mechanics methods ͑density dunctional theory or Møller-Plesset second order perturbation theory͒ while the solvent structure was obtained from Molecular Dynamics calculations. The method is very efficient; the increase in computation time is minimal with respect to previous ASEP/MD versions that worked at a fixed geometry. Despite the use of the mean field approximation in the calculation of the solvent reaction potential the agreement with previous theoretical calculations was satisfactory. Large changes were observed in the solute charge distribution induced by the solvent, and the solute polarization was accompanied by an increase in the solvent structure around the solute.