Nonadiabatic transition state theory (NA-TST) is a powerful tool to investigate the nonradiative transitions between electronic states with different spin multiplicities. The statistical nature of NA-TST provides an elegant and computationally inexpensive way to calculate the rate constants for intersystem crossings, spin-forbidden reactions, and spin-crossovers in large complex systems. The relations between the microcanonical and canonical versions of NA-TST and the traditional transition state theory are shown, followed by a review of the basic steps in a typical NA-TST rate constant calculation. These steps include evaluations of the transition probability and coupling between electronic states with different spin multiplicities, a search for the minimum energy crossing point (MECP), and computing the densities of states and partition functions for the reactant and MECP structures. The shortcomings of the spin-diabatic version of NA-TST related to ill-defined state coupling and state counting are highlighted. In three examples, we demonstrate the application of NA-TST to intersystem crossings in the active sites of metal-sulfur proteins focusing on [NiFe]-hydrogenase, rubredoxin, and Fe 2 S 2 -ferredoxin.

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.

The rational engineering of photoresponsive materials, e.g., light-driven molecular motors, is a challenging task. Here, we use structure-related design rules to prepare a prototype molecular rotary motor capable of completing an entire revolution using, exclusively, the sequential absorption of two photons; i.e., a photon-only two-stroke motor. The mechanism of rotation is then characterised using a combination of non-adiabatic dynamics simulations and transient absorption spectroscopy measurements. The results show that the rotor moiety rotates axially relative to the stator and produces, within a few picoseconds at ambient T, an intermediate with the same helicity as the starting structure. We discuss how such properties, that include a 0.25 quantum efficiency, can help overcome the operational limitations of the classical overcrowded alkene designs.

Teaching fundamental physical chemistry concepts such as the potential energy surface, transition state, and reaction path is a challenging task. The traditionally used oversimplified 2D representation of potential and free energy surfaces makes this task even more difficult and often confuses students. We show how this 2D representation can be expanded to more realistic potential and free energy surfaces by creating surface models using 3D printing technology. The printed models include potential energy surfaces for the hydrogen exchange reaction and for rotations of methyl groups in 1-fluoro-2-methylpropene calculated using quantum chemical methods. We also present several model surfaces created from analytical functions of two variables. These models include a free energy surface for protein folding, and potential energy surfaces for a linear triatomic molecule and surface adsorption, as well as simple double minimum, quadruple minimum, and parabolic surfaces. We discuss how these 3D models can be used in teaching different chemical kinetics, dynamics, and vibrational spectroscopy concepts including the potential energy surface, transition state, minimum energy reaction path, reaction trajectory, harmonic frequency, and anharmonicity.

We investigate the effect of H2 binding on the spin-forbidden nonadiabatic transition probability between the lowest energy singlet and triplet electronic states of [NiFe]-hydrogenase active site model, using a velocity averaged Landau-Zener theory. Density functional and multireference perturbation theories were used to provide parameters for the Landau-Zener calculations. It was found that variation of the torsion angle between the terminal thiolate ligands around the Ni center induces an intersystem crossing between the lowest energy singlet and triplet electronic states in the bare active site and in the active site with bound H2. Potential energy curves between the singlet and triplet minima along the torsion angle and H2 binding energies to the two spin states were calculated. Upon H2 binding to the active site, there is a decrease in the torsion angle at the minimum energy crossing point between the singlet and triplet states. The probability of nonadiabatic transitions at temperatures between 270 and 370 K ranges from 35% to 32% for the active site with bound H2 and from 42% to 38% for the bare active site, thus indicating the importance of spin-forbidden nonadiabatic pathways for H2 binding on the [NiFe]-hydrogenase active site.