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Radical organic ions can be stabilized by complexation with neutral organics via interactions that can resemble chemical bonds, but with much diminished bond energies. Those interactions are a key factor in cluster growth and polymerization reactions in ionizing environments such as regions of the interstellar medium and solar nebulae. Such radical cation complexes between naphthalene (Naph) and pyridine (Pyr) are characterized using mass-selected ion mobility experiments. The measured enthalpy of binding of the Naph+•(Pyr) heterodimer (20.9 kcal/mol) exceeds that of the Naph+•(Naph) homodimer (17.8 kcal/mol). The addition of 1-3 more pyridine molecules to the Naph+•(Pyr) heterodimer gives 10-11 kcal/mol increments in binding enthalpy. A rich array of Naph+•(Pyr) isomers are characterized by electronic structure calculations. The calculated Boltzmann distribution at 400 K yields an enthalpy of binding in reasonable agreement with experiment. The global minimum is a distonic cation formed by Pyr attack on Naph+• at the α-carbon, changing its hybridization from sp2 to distorted sp3. The measured collision cross section in helium for the Naph+•(Pyr) heterodimer of 84.9 ± 2.5 Å2 at 302 K agrees well with calculated angle-averaged cross sections (83.9-85.1 Å2 at 302 K) of the lowest energy distonic structures. A remarkable 16 kcal/mol increase in the binding energy between Naph+•(Pyr) and Bz+•(Pyr) (Bz is benzene) is understood by energy decomposition analysis. A similar increase in binding from Naph+•(NH3) to Naph+•(Pyr) (as well as between Bz+•(NH3) and Bz+•(Pyr)) is likewise rationalized.

The emergence of oxygenic photosynthesis was a major event in Earth's evolutionary history and was facilitated by chlorophylls (a major category of photopigments). The accurate modelling of photopigments is important to understand the characteristics of putative extraterrestrial life and its spectral signatures (detectable by future telescopes). In this paper, we perform a detailed assessment of various time-dependent density-functional theory (TD-DFT) methods for predicting the absorption spectra of chlorophyll a, with particular emphasis on modern low-cost approximations. We also investigate a potential extraterrestrial photopigment called phot0 and demonstrate that the electronegativity of the metal ion may exert a direct influence on the locations of the absorption peaks, with higher electronegativity inducing blue-shifting and vice-versa. Based on these calculations, we established that global-hybrid approximations with a moderate percentage of exact exchange – such as M06 and PW6B95 – are the most appropriate compromise between cost and accuracy for the computational characterisation of photopigments of astrobiological interest. We conclude with a brief assessment of the implications and avenues for future research. [ABSTRACT FROM AUTHOR]

The binding energy of the naphthalene(+•)(benzene) heterodimer cation has been determined to be 7.9 ± 1 kcal/mol for C10H8(+•)(C6H6) and 8.1 ± 1 kcal/mol for C10H8(+•)(C6D6) by equilibrium thermochemical measurements using the mass-selected drift cell technique. A second benzene molecule binds to the C10H8(+•)(C6D6) dimer with essentially the same energy (8.4 ± 1 kcal/mol), suggesting that the two benzene molecules are stacked on opposite sides of the naphthalene cation in the (C6D6)C10H8(+•)(C6D6) heterotrimer. The lowest-energy isomers of the C10H8(+•)(C6D6) and (C6D6)C10H8(+•)(C6D6) dimer and trimer calculated using the M11/cc-pVTZ method have parallel stacked structures with enthalpies of binding (-ΔH°) of 8.4 and 9.0 kcal/mol, respectively, in excellent agreement with the experimental values. The stacked face-to-face class of isomers is calculated to have substantial charge-transfer stabilization of about 45% of the total interaction energy despite the large difference between the ionization energies of benzene and naphthalene. Similarly, significant delocalization of the positive charge is found among all three fragments of the (C6D6)C10H8(+•)(C6D6) heterotrimer, thus leaving only 46% of the total charge on the central naphthalene moiety. This unexpectedly high charge-transfer component results in activating two benzene molecules in the naphthalene(+•)(benzene)2 heterotrimer cation to associate with a third benzene molecule at 219 K to form a benzene trimer cation and a neutral naphthalene molecule. The global minimum of the C10H8(+•)(C6H6)2 heterotrimer is found to be the one where the naphthalene cation is sandwiched between two benzene molecules. It is remarkable, and rather unusual, that the binding energy of the second benzene molecule is essentially the same as that of the first. This is attributed to the enhanced charge-transfer interaction in the stacked trimer radical cation.

A summary of the technical advances that are incorporated in the fourth major release of the Q-Chem quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and open-shell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller-Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly correlated Cr2 dimer, exploring zeolite-catalysed ethane dehydrogenation, energy decomposition analysis of a charged ter-molecular complex arising from glycerol photoionisation, and natural transition orbitals for a Frenkel exciton state in a nine-unit model of a self-assembling nanotube.

The stability of the five main isomers of C6H3(+) was investigated using quantum chemical calculations. The cyclic isomers are stabilized by two complementary aromatic effects, first 6-electron π aromaticity, and second a more unusual three-center two-electron σ aromaticity. Two cyclic isomers sit at the bottom of the potential energy surface with energies very close to each other, with a third cyclic isomer slightly higher. The reaction barriers for the interconversion of these isomers, as well as to convert to low-energy linear isomers, are found to be very high with transition states that break both the π and the σ aromaticities. Finally, possibilities for forming the cyclic isomers via association reactions are discussed.

The stability of the five main isomers of C6H3(+) was investigated using quantum chemical calculations. The cyclic isomers are stabilized by two complementary aromatic effects, first 6-electron π aromaticity, and second a more unusual three-center two-electron σ aromaticity. Two cyclic isomers sit at the bottom of the potential energy surface with energies very close to each other, with a third cyclic isomer slightly higher. The reaction barriers for the interconversion of these isomers, as well as to convert to low-energy linear isomers, are found to be very high with transition states that break both the π and the σ aromaticities. Finally, possibilities for forming the cyclic isomers via association reactions are discussed.

In the past decade, a number of approaches have been developed to fix the failure of (semi)local density-functional theory (DFT) in describing intermolecular interactions. The performance of several such approaches with respect to highly accurate benchmarks is compared here on a set of separation-dependent interaction energies for ten dimers. Since the benchmarks were unknown before the DFT-based results were collected, this comparison constitutes a blind test of these methods. [ABSTRACT FROM AUTHOR]

Counting parameters has become customary in the density functional theory community as a way to infer the transferability of popular approximations to the exchange‐correlation functionals. Recent work in data science, however, has demonstrated that the number of parameters of a fitted model is not related to the complexity of the model itself, nor to its eventual overfitting. Using similar arguments, here, we show that it is possible to represent every modern exchange‐correlation functional approximations using just one single parameter. This procedure proves the futility of the number of parameters as a measure of transferability. To counteract this shortcoming, we introduce and analyze the performance of three statistical criteria for the evaluation of the transferability of exchange‐correlation functionals. The three criteria are called Akaike information criterion, Vapnik‐Chervonenkis criterion, and cross‐validation criterion and are used in a preliminary assessment to rank 60 exchange‐correlation functional approximations using the ASCDB database of chemical data. [ABSTRACT FROM AUTHOR]

We report a test of 30 density functionals, including several recent ones, for their predictions of 69 singlet-to-singlet excitation energies of 11 molecules. The reference values are experimental results collected by Caricato et al. for 30 valence excitations and 39 Rydberg excitations. All calculations employ time-dependent density functional theory in the adiabatic, linear-response approximation. As far as reasonable, all of the assignments are performed by essentially the same protocol as used by Caricato et al., and this allows us to merge our mean unsigned errors (MUEs) with the ones they calculated for both density functional and wave function methods. We find 21 of the 30 density functionals calculated here have smaller MUEs for the 30 valence states than what they obtained (0.47 eV) for the state-of-the-art EOM-CCSD wave function. In contrast, for all of density functionals the MUE for 39 Rydberg states is larger than that (0.11 eV) of EOM-CCSD. Merging the 30 density functionals calculated here with the 26 calculated by Caricato et al. makes a set of 56 density functionals. Averaging the unsigned errors over both the valence excitations and the Rydberg excitations, none of the 56 density functionals shows a lower mean unsigned error than that (0.27 eV) of EOM-CCSD. Nevertheless, two functionals are successful in having an overall mean unsigned error of 0.30 eV, and another nine are moderately successful in having overall mean unsigned errors in the range 0.32-0.36 eV. Successful or moderately successful density functionals include seven hybrid density functionals with 41% to 54% Hartree-Fock exchange, and four range-separated hybrid density functionals in which the percentage of Hartree-Fock exchange increases from 0% to 19% at small interelectronic separation to 65%-100% at long range. [ABSTRACT FROM AUTHOR]

We extend our recent SOGGA11 approximation to the exchange-correlation functional to include a percentage of Hartree-Fock exchange. The new functional, called SOGGA11-X, has better overall performance for a broad chemical database than any previously available global hybrid generalized gradient approximation, and in addition it satisfies an extra physical constraint in that it is correct to second order in the density-gradient. [ABSTRACT FROM AUTHOR]

A database containing 17 multiplicity-changing valence and Rydberg excitation energies of p-block elements is used to test the performance of density functional theory (DFT) with approximate density functionals for calculating relative energies of spin states. We consider only systems where both the low-spin and high-spin state are well described by a single Slater determinant, thereby avoiding complications due to broken-symmetry solutions. Because the excitations studied involve a spin change, they require a balanced treatment of exchange and correlation, thus providing a hard test for approximate density functionals. We test three formalisms for predicting the multiplicity-changing transition energies. First is the ΔSCF method; we also test time-dependent density functional theory (TDDFT), both in its conventional form starting from the low-spin state and in its collinear spin-flip form starting from the high-spin state. Very diffuse basis functions are needed to give a qualitatively correct description of the Rydberg excitations. The scalar relativistic effect needs to be considered when quantitative results are desired, and we include it in the comparisons. With the ΔSCF method, most of the tested functionals give mean unsigned errors (MUEs) larger than 6 kcal/mol for valence excitations and MUEs larger than 3 kcal/mol for Rydberg excitations, but the performance for the Rydberg states is much better than can be obtained with time-dependent DFT. It is surprising to see that the long-range corrected functionals, which have 100% Hartree-Fock exchange at large inter-electronic distance, do not improve the performance for Rydberg excitations. Among all tested density functionals, ΔSCF calculations with the O3LYP, M08-HX, and OLYP functionals give the best overall performance for both valence and Rydberg excitations, with MUEs of 2.1, 2.6, and 2.7 kcal/mol, respectively. This is very encouraging since the MUE of the CCSD(T) coupled cluster method with quintuple zeta basis sets is 2.0 kcal/mol; however, caution is advised since many popular density functionals give poor results, and there can be very significant differences between the ΔSCF predictions and those from TDDFT. [ABSTRACT FROM AUTHOR]

The importance of databases of reliable and accurate data in chemistry has substantially increased in the past two decades. Their main usage is to parametrize electronic structure theory methods, and to assess their capabilities and accuracy for a broad set of chemical problems. The collection we present here—ACCDB—includes data from 16 different research groups, for a total of 44,931 unique reference data points, all at a level of theory significantly higher than density functional theory, and covering most of the periodic table. It is composed of five databases taken from literature (GMTKN, MGCDB84, Minnesota2015, DP284, and W4‐17), two newly developed reaction energy databases (W4‐17‐RE and MN‐RE), and a new collection of databases containing transition metals. A set of expandable software tools for its manipulation is also presented here for the first time, as well as a case study where ACCDB is used for benchmarking commercial CPUs for chemistry calculations. © 2018 Wiley Periodicals, Inc. ACCDB—A Collection of Chemistry DataBases—includes five previously published databases from different sources (GMTKN, MGCDB84, DP284, Minnesota2015B, and W4‐17), and three new one (W4‐17‐RE, MN‐RE, and Metals&EE), and it includes 44,931 unique reference data (all at a level significantly higher than DFT), and 10,049 structures. Software tools for manipulation of ACCDB are also presented, as well as a case study for the performance of commercial CPUs for quantum chemistry calculations. [ABSTRACT FROM AUTHOR]