Nonadiabatic mixed quantum-classical (NA-MQC) dynamics methods form a class of computational theoretical approaches in quantum chemistry tailored to investigate the time evolution of nonadiabatic phenomena in molecules and supramolecular assemblies. NA-MQC is characterized by a partition of the molecular system into two subsystems: one to be treated quantum mechanically (usually but not restricted to electrons) and another to be dealt with classically (nuclei). The two subsystems are connected through nonadiabatic couplings terms to enforce self-consistency. A local approximation underlies the classical subsystem, implying that direct dynamics can be simulated, without needing precomputed potential energy surfaces. The NA-MQC split allows reducing computational costs, enabling the treatment of realistic molecular systems in diverse fields. Starting from the three most well-established methods-mean-field Ehrenfest, trajectory surface hopping, and multiple spawning-this review focuses on the NA-MQC dynamics methods and programs developed in the last 10 years. It stresses the relations between approaches and their domains of application. The electronic structure methods most commonly used together with NA-MQC dynamics are reviewed as well. The accuracy and precision of NA-MQC simulations are critically discussed, and general guidelines to choose an adequate method for each application are delivered.

The Newton-X program is a general-purpose program package for excited-state molecular dynamics, including nonadiabatic methods. Its modular design allows Newton-X to be easily linked to any quantum-chemistry package that can provide excited-state energy gradients. At the current version, Newton-X can perform nonadiabatic dynamics using Columbus, Turbomole, Gaussian, and Gamess program packages with multireference configuration interaction, multiconfigurational self-consistent field, time-dependent density functional theory, and other methods. Nonadiabatic dynamics simulations with a hybrid combination of methods, such as Quantum-Mechanics/Molecular-Mechanics, are also possible. Moreover, Newton-X can be used for the simulation of absorption and emission spectra. The code is distributed free of charge for noncommercial and nonprofit uses at www.newtonx.org

A comprehensive effort in photodynamical ab initio simulations of the ultrafast deactivation pathways for all five nucleobases adenine, guanine, cytosine, thymine, and uracil is reported. These simulations are based on a complete nonadiabatic surface-hopping approach using extended multiconfigurational wave functions. Even though all five nucleobases share the basic internal conversion mechanisms, the calculations show a distinct grouping into purine and pyrimidine bases as concerns the complexity of the photodynamics. The purine bases adenine and guanine represent the most simple photodeactivation mechanism with the dynamics leading along a diabatic ππ* path directly and without barrier to the conical intersection seam with the ground state. In the case of the pyrimidine bases, the dynamics starts off in much flatter regions of the ππ* energy surface due to coupling of several states. This fact prohibits a clear formation of a single reaction path. Thus, the photodynamics of the pyrimidine bases is much richer and includes also nπ* states with varying importance, depending on the actual nucleobase considered. Trapping in local minima may occur and, therefore, the deactivation time to the ground state is also much longer in these cases. Implications of these findings are discussed (i) for identifying structural possibilities where singlet/ triplet transitions can occur because of sufficient retention time during the singlet dynamics and (ii) concerning the flexibility of finding other deactivation pathways in substituted pyrimidines serving as candidates for alternative nucleobases.photodynamical simulation | photostability | ultrafast photodeactivation | nonadiabatic interactions | ab initio multireference methods O wing to the importance of mutagenic and carcinogenic effects caused by UV radiation on DNA, the UV-induced photochemistry and photophysics of individual bases (1-7), base pairs (8-10), and nuclei acid strands (11-13) have been intensively studied. In each of these levels, it has been found that the genetic code may count on molecular mechanisms to get rid of the energy excess minimizing deleterious effects. Isolated nucleobases (Scheme 1), for instance, are all photostable by returning to the ground state in an ultrafast time scale of a few picoseconds (1, 2, 14-20), which reduces the probability of undergoing photochemical transformations induced by reactive excited states. The ultrafast deactivation of the five natural nucleobases contrasts with the long lifetime of analogous bases not found in DNA and RNA (2, 21), suggesting that it could have been a source of evolutionary pressure in early biotic ages.Ultrafast deactivation occurs through internal conversion mechanisms, where the molecule transfers the photoenergy stored in the electronic system to nuclear vibrational degrees of freedom. This means, the deactivation takes place without photoemission in contrast to what occurs for fluorescent and phosphorescent species. The energy transfer proceeds more intensely near nuclear geometries with stat...