Simple, Efficient, and Universal Energy Decomposition Analysis Method Based on Dispersion-Corrected Density Functional Theory.

Energy decomposition analysis (EDA) is an important class of methods to explore the nature of interaction between fragments in a chemical system. It can decompose the interaction energy into different physical components to understand the factors that play key roles in the interaction. This work proposes an EDA strategy based on dispersion-corrected density functional theory (DFT), called sobEDA. This method is fairly easy to implement and very universal. It can be used to study weak interactions, chemical bond interactions, open-shell systems, and interactions between multiple fragments. The total time consumption of sobEDA is only about twice that of conventional DFT single-point calculation for the entire system. This work also proposes a variant of the sobEDA method named sobEDAw, which is designed specifically for decomposing weak interaction energies. Through a proper combination of DFT correlation energy and dispersion correction term, sobEDAw gives a ratio between dispersion energy and electrostatic energy that is highly consistent with the symmetry-adapted perturbation theory, which is quite popular and robust in studying weak interactions but expensive. We present a shell script sobEDA.sh to implement the methods proposed in this work based on the very popular Gaussian quantum chemistry program and Multiwfn wavefunction analysis code. Via the script, theoretical chemists can use the sobEDA and sobEDAw methods very conveniently in their study. Through a series of examples, the rationality of the new methods and their implementation are verified, and their great practical values in the study of various chemical systems are demonstrated.

Independent gradient model based on Hirshfeld partition: A new method for visual study of interactions in chemical systems.

The powerful independent gradient model (IGM) method has been increasingly popular in visual analysis of intramolecular and intermolecular interactions in recent years. However, we frequently observed that there is an evident shortcoming of IGM map in graphically studying weak interactions, that is its isosurfaces are usually too bulgy; in these cases, not only the graphical effect is poor, but also the color on some areas on the isosurfaces is inappropriate and may lead to erroneous analysis conclusions. In addition, the IGM method was originally proposed based on promolecular density, which is quite crude and does not take actual electronic structure into account. In this article, we propose an improvement version of IGM, namely IGM based on Hirshfeld partition of molecular density (IGMH), which replaces the free-state atomic densities involved in the IGM method with the atomic densities derived by Hirshfeld partition of actual molecular electron density. This change makes IGM have more rigorous physical background. A large number of application examples in this article, including molecular and periodic systems, weak and chemical bond interactions, fully demonstrate the important value of IGMH in intuitively understanding interactions in chemical systems. Comparisons also showed that the IGMH usually has markedly better graphical effect than IGM and overcomes known problems in IGM. Currently IGMH analysis has been supported in our wavefunction analysis code Multiwfn (http://sobereva.com/multiwfn). We hope that IGMH will become a new useful method among chemists for exploring interactions in wide variety of chemical systems.

Remarkable Size Effect on Photophysical and Nonlinear Optical Properties of All-Carboatomic Rings, Cyclo[18]carbon and Its Analogues.

Inspired by recent experimental observation of molecular morphology and theoretical predictions of multiple properties of cyclo[18]carbon, we systematically studied the photophysical and nonlinear optical properties of cyclo[2N]carbons (N=3-15) allotropes through density functional theory. This work unveils the unusual optical properties of the sp-hybridized carbon rings with different sizes. The remarkable size dependence of the optical properties of these systems and their underlying nature are profoundly explored, and the relevance between aromaticity and optical properties are highlighted. The extrapolation curves fitted for energy level of frontier molecular orbitals, maximum absorption wavelength, and (hyper)polarizability of considered carbon rings are presented, which can be used to reliably predict corresponding properties for arbitrarily large carbon rings. The findings in this study will facilitate the exploration of potential application of cyclocarbons in the field of optical materials.

Comment on "Theoretical investigation on bond and spectrum of cyclo[18]carbon (C18) with sp-hybridized".

Recently, Shuhong Xu et al. reported theoretical calculation of molecular structure, bonding, aromaticity, electron delocalization, and electronic spectrum of cyclo[18]carbon in J. Mol. Model., 26, 111 (2020). Due to inappropriate consideration of calculation strategy, misunderstanding of some analysis methods and concepts, and some errors in the data, there are misleading statements and unconvincing conclusions in their paper. Here, we will point out inadequacies of Shuhong Xu's paper and put forward our own views. The contents of this comment will also help those who are studying cyclo[18]carbon to better understand this system and its analogues.

Ultrastrong Regulation Effect of the Electric Field on the All-Carboatomic Ring Cyclo[18]Carbon*.

Cyclo[18]carbon has a very unique geometry and electronic structure. We found that an external electric field (EEF) has an ultrastrong regulation effect on various aspects of the cyclo[18]carbon: (1) The EEF makes the shape of the cyclo[18]carbon change from a circle to an oval, the elongation is particularly striking at a large EEF magnitude. (2) The EEF causes a huge polarization of distribution of in-plane π electrons, and strong EEF can even make some of the electrons detached from the carbon ring (3) EEF significantly lowers LUMO energy and reduces HOMO-LUMO gap (4) Large EEF leads to absorption band in the visible light range and thus makes the cyclo[18]carbon display color (5) Strong EEF causes a large number of new absorption peaks in IR spectrum. We also carefully analyzed how EEF deforms structure of the cyclo[18]carbon from the perspective of atomic forces and decomposition of energy variation, and the reason why the in-plane π electrons are much more polarizable by EEF than the out-of-plane π electrons is discussed. Moreover, we demonstrated that it is feasible to equivalently apply a strong EEF on the cyclo[18]carbon via a purely chemical and thus a more easily achieve way, namely introducing divalent alkaline earth metal cation.

Vibrational Spectra and Molecular Vibrational Behaviors of All-Carboatomic Rings, cyclo[18]carbon and Its Analogues.

The vibrational spectra of cyclo[18]carbon and its analogues, cyclo[2n]carbon (n=3 to 15), were carefully simulated and characterized. The in-plane C-C stretching vibrations shows strong rigidity, while out-of-plane motions seem to be extremely flexible. The solvation effect can enhance signal strengths of the vibrational spectra, but does not evidently change the shape of the spectral curves. The infrared and Raman spectra of cyclo[2n]carbons are quite sensitive to ring size in the range of n=3 to 7, while the size only modestly affects peak positions and strengths for larger rings. Molecular dynamic trajectories show that the fluctuation period of the skeleton of cyclo[18]carbon is basically constant at different temperatures, and they are all about 300 fs. With increase of simulation temperature, the ring distortion due to thermal motion is notable and becomes much stronger. However, neither ring breaking nor isomerization in cyclo[18]carbon is observed during the simulations untill 298.15 K.

van der Waals potential: an important complement to molecular electrostatic potential in studying intermolecular interactions.

Electrostatics and van der Waals (vdW) interactions are two major components of intermolecular weak interactions. Electrostatic potential has been a very popular function in revealing electrostatic interaction between the system under study and other species, while the role of vdW potential was less recognized and has long been ignored. In this paper, we explicitly present definition of vdW potential, describe its implementation details, and demonstrate its important practical values by several examples. We hope this work can arouse researchers' attention to the vdW potential and promote its application in the studies of weak interactions. Calculation, visualization, and quantitative analysis of the vdW potential have been supported by our freely available code Multiwfn ( http://sobereva.com/multiwfn ).

Influence of Torsional Anharmonicity on the Reactions of Methyl Butanoate with Hydroperoxyl Radical.

An ab initio chemical kinetics study of the reactions of methyl butanoate (MB) with hydroperoxyl radical (HO2) is presented in this paper. Particular interest is placed on determining the influences of torsional anharmonicity and addition reaction on the rate constants of hydrogen abstraction reactions. Stationary points on the potential energy surface of MB + HO2 are calculated at the level of QCISD(T)/CBS//B3LYP/6-311++G(d,p). The transition state theory (TST) is used to calculate the high-pressure limit rate constants of the hydrogen abstraction reactions over a board range of temperature (500-2000 K). Anharmonicity of low-frequency torsional modes is considered in the rate calculations by using the one-dimensional hindered rotor approximation and the internal-coordinate multistructural approximation; the latter is used as a higher-level theoretical method to examine the applicability of the former in dealing with strongly coupled torsional modes. The calculated rate constants are compared with the available data from the literature and observed discrepancies are analyzed in detail. An energetically lowest-lying addition reaction with subsequent isomerization and decomposition reactions are identified on the potential energy surface. The multiple-well Master equation analysis shows that these reactions have a secondary influence on the rate constants in the temperature range of interest.