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Mechanistic investigations at the density functional theory level of organic and organometallic reactions in solution are now broadly accessible and routinely implemented to complement experimental investigations. The selection of an appropriate functional among the plethora of developed ones is the first challenge on the way to reliable energy barrier calculations. To provide guidelines for the choice of an initial and reliable computational level, the performances of commonly used non‐empirical (PBE, PBE0, PBE0‐DH) and empirical density functionals (BLYP, B3LYP, B2PLYP) were evaluated relative to experimental activation enthalpies. Most reactivity databases to assess density functional performances are primarily based on high level calculations, here a set of experimental activation enthalpies of organic and organometallic reactions performed in solution were selected from the literature. As a general trend, the non‐empirical functionals outperform the empirical ones. The most accurate energy barriers are obtained with hybrid PBE0 and double‐hybrid PBE0‐DH density functionals, both providing similar performance. Regardless of the functional under consideration, the addition of the GD3‐BJ empirical dispersion correction does not enhance the accuracy of computed energy barriers. [ABSTRACT FROM AUTHOR]

Setting up computational approaches enabling the correct prediction of the photophysical properties of rhodamine B (RB) derivatives and their aggregates (referred to as dimers here) is of fundamental importance to rationally drive the design of novel systems of applicative relevance, such as artificial light-harvesting nanosystems. Currently, approaches rooted in time-dependent density functional theory (TD-DFT), which are appealing for their relatively low computational cost, nonetheless have limitations in terms of accuracy, especially while considering RB dimeric species. In this work, we investigated the performances of optimally tuned range-separated hybrid functionals for describing the excited states of RB and its H dimer, focusing on dimeric charge-transfer (CT) states. We compared different optimal tuning (OT) procedures including or not solvent screening. The results show that the properties of CT states, such as their CT extent, brightness and relative energy ordering, are crucially affected by the Hartree–Fock exchange amount dictated by OT, which is in turn driven by the chosen asymptotic behavior. Finally, to understand the dissimilar pictures provided for the CT states by different tuning approaches, we performed an extensive analysis aimed at elucidating how CT states are affected by range-separation parameters. As a result, a simple procedure is finally provided for easily achieving a unique functional to be applied on both monomer and dimers, its current limitations are highlighted and possible perspectives for future development are envisaged. [ABSTRACT FROM AUTHOR]

Transition Metal Complexes (TMCs) are known for the rich variety of their excited states showing different nature and degrees of locality. Describing the energies of these excited states with the same degree of accuracy is still problematic when using time-dependent density functional theory in conjunction with the most current density functional approximations. In particular, the presence of unphysically low lying excited states possessing a relevant Charge Transfer (CT) character may significantly affect the spectra computed at such a level of theory and, more relevantly, the interpretation of their photophysical behavior. In this work, we propose an improved version of the MAC index, recently proposed by the authors and collaborators, as a simple and computationally inexpensive diagnostic tool that can be used for the detection and correction of the unphysically predicted low lying excited states. The analysis, performed on five prototype TMCs, shows that spurious and ghost states can appear in a wide spectral range and that it is difficult to detect them only on the basis of their CT extent. Indeed, both delocalization of the excited state and CT extent are criteria that must be combined, as in the MAC index, to detect unphysical states. [ABSTRACT FROM AUTHOR]

Organic luminophores displaying one or more forms of luminescence enhancement in solid state are extremely promising for the development and performance optimization of functional materials essential to many modern key technologies. Yet, the effort to harness their huge potential is riddled with hurdles that ultimately come down to a limited understanding of the interactions that result in the diverse molecular environments responsible for the macroscopic response. In this context, the benefits of a theoretical framework able to provide mechanistic explanations to observations, supported by quantitative predictions of the phenomenon, are rather apparent. In this perspective, we review some of the established facts and recent developments about the current theoretical understanding of solid-state luminescence enhancement (SLE) with an accent on aggregation-induced emission (AIE). A description of the macroscopic phenomenon and the questions it raises is accompanied by a discussion of the approaches and quantum chemistry methods that are more apt to model these molecular systems with the inclusion of an accurate yet efficient simulation of the local environment. A sketch of a general framework, building from the current available knowledge, is then attempted via the analysis of a few varied SLE/AIE molecular systems from literature. A number of fundamental elements are identified offering the basis for outlining design rules for molecular architectures exhibiting SLE that involve specific structural features with the double role of modulating the optical response of the luminophores and defining the environment they experience in solid state. [ABSTRACT FROM AUTHOR]

This document is a preface for a virtual special issue of the Journal of Computational Chemistry (JCC) celebrating the 60th birthday and scientific career of Carlo Adamo. Carlo is a theoretical chemist who has made significant contributions to the field, particularly in the development of Density Functional Theory (DFT) and the PBE0 functional. He has published extensively and has been recognized for his research excellence. In addition to his scientific pursuits, Carlo is also passionate about mountaineering and has maintained long-standing collaborations and friendships throughout his career. The preface expresses gratitude to all contributors and provides a brief overview of Carlo's background and achievements. [Extracted from the article]