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The amount of data and number of databases for the assessment and parameterization of density functional theory methods has grown substantially in the past two decades. In this work, we introduce a novel cluster analysis technique for density functional theory calculations of the electronic structure of atoms and molecules with the goal of creating new statistically significant databases with broad chemical scope, and a manageable number of data-points. By analyzing without a priori chemical assumptions a population of almost 350k data-points, we create a new database called ASCDB containing only 200 data-points. This new database holds the same chemical information as the larger population of data from which it is obtained, but with a computational cost that is reduced by several orders of magnitude. The labelling of the significant chemical properties is performed a posteriori on the resulting 16 subsets, classifying them into four areas of chemical importance: non-covalent interactions, thermochemistry, non-local effects, and unbiased calculations. The analysis of the results and their transferability shows that ASCDB is capable of providing the same information as that of the larger collection of data-such as GMTKN55, MGCDB84, and Minnesota 2015B-for several density functional theory methods and basis sets. In light of these results, we suggest the use of this new small database as a first inexpensive tool for the evaluation and parameterization of electronic structure theory methods.

We seek insight into the origin of observations made in plasma experiments mimicking interstellar and circumstellar conditions. To this end theory is applied to the low-energy isomers of C4H2(+), C6H2(+) and C6H4(+) and their formation paths from acetylene and its fragments. Ab initio molecular dynamics trajectories are performed to explore which isomers are readily accessible from acetylene and its ion fragments. Structural information at a high level of electronic structure theory [CCSD(T)/cc-pVTZ], as well as information on the vibrational [UMP2] and electronic spectra [ωB97X] of the low-energy isomers is reported.

We present two new exchange-correlation functionals for hybrid Kohn-Sham electronic structure calculations based on the nonseparable functional form introduced recently in the N12 and MN12-L functionals but now with the addition of screened Hartree-Fock exchange. The first functional depends on the density and the density gradient and is called N12-SX; the second functional depends on the density, the density gradient, and the kinetic energy density and is called MN12-SX. Both new functionals include a portion of the Hartree-Fock exchange at short-range, but Hartree-Fock exchange is screened at long range. The accuracies of the two new functionals are compared to those of the recent N12 and MN12-L local functionals to show the effect of adding screened exchange, are compared to the previously best available screened exchange functional, HSE06, and are compared to the best available global-hybrid generalized gradient approximation (GGA) and to a high-performance long-range-corrected meta-GGA.

We report a new local exchange-correlation energy functional that has significantly improved across-the-board performance, including main-group and transition metal chemistry and solid-state physics, especially atomization energies, ionization potentials, barrier heights, noncovalent interactions, isomerization energies of large moleucles, and solid-state lattice constants and cohesive energies.

Adiabatic time-dependent density functional theory is a powerful method for calculating electronic excitation energies of complex systems, but the quality of the results depends on the choice of approximate density functional. In this article we test two promising new density functionals, M11 and M11-L, against databases of 214 diverse electronic excitation energies, and we compare the results to those for 16 other density functionals of various kinds and to time-dependent Hartree-Fock. Charge transfer excitations are well known to be the hardest challenge for TDDFT. M11 is a long-range-corrected hybrid meta-GGA, and it shows better performance for charge transfer excitations than any of the other functionals except M06-HF, which is a specialized functional that does not do well for valence excitations. Several other long-range-corrected hybrid functionals also do well, and we especially recommend M11, ωB97X, and M06-2X for general spectroscopic applications because they do exceptionally well on ground-state properties as well as excitation energies. Local functionals are preferred for many applications to extended systems because of their significant cost advantage for large systems. M11-L is a dual-range local functional and-unlike all previous local functionals-it has good performance for Rydberg states as well as for valence states. Thus it is highly recommended for excitation energy calculations on extended systems.

A computational investigation is carried out on the parent triaziridine and as a function of N-substituents. Assessment of heat of formation, ring strain energy, barriers to inversion of nitrogen, and NMR criteria leads to understanding of issues related to vicinal lone pair repulsion and aromatic stabilization. Results lead to the proposal of a potentially flat structure with a pi aromatic-like triaziridine system, N(3)(BH(2))(3).