Search

Electronic structure calculations have the potential to predict key matter transformations for applications of strategic technological importance, from drug discovery to material science and catalysis. However, a predictive physicochemical characterization of these processes often requires accurate quantum chemical modeling of complex molecular systems with hundreds to thousands of atoms. Due to the computationally demanding nature of electronic structure calculations and the complexity of modern high-performance computing hardware, quantum chemistry software has historically failed to operate at such large molecular scales with accuracy and speed that are useful in practice. In this paper, novel algorithms and software are presented that enable extreme-scale quantum chemistry capabilities with particular emphasis on exascale calculations. This includes the development and application of the multi-Graphics Processing Unit (GPU) library LibCChem 2.0 as part of the General Atomic and Molecular Electronic Structure System package and of the standalone Extreme-scale Electronic Structure System (EXESS), designed from the ground up for scaling on thousands of GPUs to perform high-performance accurate quantum chemistry calculations at unprecedented speed and molecular scales. Among various results, we report that the EXESS implementation enables Hartree–Fock/cc-pVDZ plus RI-MP2/cc-pVDZ/cc-pVDZ-RIFIT calculations on an ionic liquid system with 623 016 electrons and 146 592 atoms in less than 45 min using 27 600 GPUs on the Summit supercomputer with a 94.6% parallel efficiency. [ABSTRACT FROM AUTHOR]

The characterization of an ionic liquid's properties based on structural information is a longstanding goal of computational chemistry, which has received much focus from ab initio and molecular dynamics calculations. This work examines kernel ridge regression models built from an experimental dataset of 2212 ionic liquid melting points consisting of diverse ion types. Structural descriptors, which have been shown to predict quantum mechanical properties of small neutral molecules within chemical accuracy, benefit from the addition of first-principles data related to the target property (molecular orbital energy, charge density profile, and interaction energy based on the geometry of a single ion pair) when predicting the melting point of ionic liquids. Out of the two chosen structural descriptors, ECFP4 circular fingerprints and the Coulomb matrix, the addition of molecular orbital energies and all quantum mechanical data to each descriptor, respectively, increases the accuracy of surrogate models for melting point prediction compared to using the structural descriptors alone. The best model, based on ECFP4 and molecular orbital energies, predicts ionic liquid melting points with an average mean absolute error of 29 K and, unlike group contribution methods, which have achieved similar results, is applicable to any type of ionic liquid. [ABSTRACT FROM AUTHOR]

The prediction of physicochemical properties of ionic liquids such as conductivity and melting point would substantially aid the targeted design of ionic liquids for specific applications ranging from solvents for extraction of valuable chemicals to biowaste to electrolytes in alternative energy devices. The previously published study connecting the interaction energies of single ion pairs (1 IP) of ionic liquids to their thermodynamic and transport properties has been extended to larger systems consisting of two ion pairs (2 IPs), in which many-body and same-ion interactions are included. Routinely used cations, of the imidazolium and pyrrolidinium families, were selected in the study coupled with chloride, tetrafluoroborate, and dicyanamide. Their two ion pair clusters were subjected to extensive configuration screening to establish most stable structures. Interaction energies of these clusters were calculated at the spin-ratio scaled MP2 (SRS-MP2) level for the correlation interaction energy, and a newly developed scaled Hartree-Fock method for the rest of energetic contributions to interaction energy. A full geometry screening for each cation-anion combination resulted in 192 unique structures, whose stability was assessed using two criteria—widely used interaction energy and total electronic energy. Furthermore, the ratio of interaction energy to its dispersion component was correlated with experimentally observed melting points in 64 energetically favourable structures. These systems were also used to test the correlation of the dispersion contribution to interaction energy with measured conductivity. [ABSTRACT FROM AUTHOR]

Dinuclear silver formamidinates, [Ag2(DFForm)2(py)2] (1) (DFForm, N , N ′-bis(2,6-difluorophenyl)formamidinate, py = pyridine), [Ag2(m HForm)2(py)3] (2) and [Ag2(m HForm)2(CH3CN)2] (2a) (m HForm, N , N ′-bis(2,3,4,6-tetrafluorophenyl)formamidinate), [Ag2(p HForm)2(py)3] (3) (p HForm, N , N ′-bis(2,3,5,6-tetrafluorophenyl)formamidinate), [Ag2(PFForm)2(py)3] (4) (PFForm, N , N ′-bis(2,3,4,5,6-pentafluorophenyl)formamidinate) and [Ag2(DippForm)2]·1.25CH2Cl2 (5) (DippForm, N , N ′-bis(2,6-diisopropylphenyl)formamidinate), and polynuclear, mainly trinuclear, silver pyrazolates, [Ag3(Ph2pz)3]2 (6) (Ph2pz, 3,5-diphenylpyrazolate), [Ag(t Bu2pz)]4 (7) (t Bu2pz, 3,5-di- tert -butylpyrazolate), [Ag3(PhMepz)3] (8) (PhMepz, 3-methyl-5-phenylpyrazolate), [Ag3(Phtpz)3] (9) (Phtpz, 3-phenyl-5-(2-thienyl)pyrazolate), [Ag3(ttfpz)3] (10) (ttfpz, 3-(2-thienyl)-5-trifluoromethylpyrazolate) and [Ag2(Me2pz)(PPh3)4Cl] (11) (Me2pz, 3,5-dimethylpyrazolate) were successfully synthesized in good yield by treating silver(i) oxide with the corresponding formamidines (FormH) or pyrazoles (pzH) either in pyridine or acetonitrile. These rapid, easily handled and economical reactions are readily monitored by consumption of black silver oxide. All compounds are potential precursors in redox transmetallation reactions in the synthesis of lanthanoid and alkaline earth pyrazolate and formamidinate complexes. Dinuclear silver formamidinates and polynuclear, mainly trinuclear, silver pyrazolates were successfully synthesized in good yield by treating silver(i) oxide with the corresponding formamidines (FormH) or pyrazoles (pzH) either in pyridine or acetonitrile. [ABSTRACT FROM AUTHOR]

A study to examine the role of ionic liquids (ILs) in electrochemical devices is conducted. The ILs is used in electrochemical devices such as solar cells, high energy density batteries, fuel cells and supercapacitors for improving sustainability of energy supply.

A study is conducted to identify and benchmark reliable low-cost theoretical procedures for studying the addition-fragmentation processes in reversible-addition-fragmentation-transfer (RAFT) polymerization is presented. The addition fragmentation reaction in the RAFT polymerization process is a difficult system to model computationally, with lower cost procedures such as ROMP2 and particularly DFT, having large absolute and relative errors for the thermodynamics of some systems.

The accurate calculation of intermolecular interactions is important to our understanding of properties in large molecular systems. The high computational cost of the current "gold standard" method, coupled cluster with singles and doubles and perturbative triples (CCSD(T), limits its application to small- to medium-sized systems. Second-order Møller–Plesset perturbation (MP2) theory is a cheaper alternative for larger systems, although at the expense of its decreased accuracy, especially when treating van der Waals complexes. In this study, a new modification of the spin-component scaled MP2 method was proposed for a wide range of intermolecular complexes including two well-known datasets, S22 and S66, and a large dataset of ionic liquids consisting of 174 single ion pairs, IL174. It was found that the spin ratio, ϵΔs=EOSINT/ESSINT, calculated as the ratio of the opposite-spin component to the same-spin component of the interaction correlation energy fell in the range of 0.1 and 1.6, in contrast to the range of 3–4 usually observed for the ratio of absolute correlation energy, ϵs=EOS/ESS, in individual molecules. Scaled coefficients were found to become negative when the spin ratio fell in close proximity to 1.0, and therefore, the studied intermolecular complexes were divided into two groups: (1) complexes with ϵΔs<<1 and (2) complexes with ϵΔs≥<1. A separate set of coefficients was obtained for both groups. Exclusion of counterpoise correction during scaling was found to produce superior results due to decreased error. Among a series of Dunning's basis sets, cc-pVTZ and cc-pVQZ were found to be the best performing ones, with a mean absolute error of 1.4 kJ mol-1 and maximum errors below 6.2 kJ mol-1. The new modification, spin-ratio scaled second-order Møller–Plesset perturbation, treats both dispersion-driven and hydrogen-bonded complexes equally well, thus validating its robustness with respect to the interaction type ranging from ionic to neutral species at minimal computational cost. [ABSTRACT FROM AUTHOR]

The accuracy of correlation energy recovered by coupled cluster single‐, double‐, and perturbative triple‐excitations, CCSD(T), has led to the method being considered the gold standard of computational chemistry. The application of CCSD(T) has been limited to medium‐sized molecular systems due to its steep scaling with molecular size. The recent development of alternative domain‐based local pair natural orbital coupled‐cluster method, DLPNO‐CCSD(T), has significantly broadened the range of chemical systems to which CCSD(T) level calculations can be applied. Condensed systems such as ionic liquids (ILs) have a large contribution from London dispersion forces of up to 150 kJ mol−1 in large‐scale clusters. Ionic liquids show appreciable charge transfer effects that result in the increased valence orbital delocalization over the entire ionic network, raising the question whether the application of methods based on localized orbitals is reliable for these semi‐Coulombic materials. Here the performance of DLPNO‐CCSD(T) is validated for the prediction of correlation interaction energies of two data sets incorporating single‐ion pairs of protic and aprotic ILs. DLPNO‐CCSD(T) produced results within chemical accuracy with tight parameter settings and a non‐iterative treatment of triple excitations. To achieve spectroscopic accuracy of 1 kJ mol−1, especially for hydrogen‐bonded ILs and those containing halides, the DLPNO settings had to be increased by two orders of magnitude and include the iterative treatment of triple excitations, resulting in a 2.5‐fold increase in computational cost. Two new sets of parameters are put forward to produce the performance of DLPNO‐CCSD(T) within chemical and spectroscopic accuracy. [ABSTRACT FROM AUTHOR]

We present an inexpensive and robust theoretical approach based on the fragment molecular orbital methodology and the spin‐ratio scaled second‐order Møller–Plesset perturbation theory to predict the lattice energy of benzene crystals within 2 kJ⋅mol−1. Inspired by the Harrison method to estimate the Madelung constant, the proposed approach calculates the lattice energy as a sum of two‐ and three‐body interaction energies between a reference molecule and the surrounding molecules arranged in a sphere. The lattice energy converges rapidly at a radius of 13 Å. Adding the corrections to account for a higher correlated level of theory and basis set superposition for the Hartree Fock (HF) level produced a lattice energy of −57.5 kJ⋅mol−1 for the benzene crystal structure at 138 K. This estimate is within 1.6 kJ⋅mol−1 off the best theoretical prediction of −55.9 kJ⋅mol−1. We applied this approach to calculate lattice energies of the crystal structures of phase I and phase II—polymorphs of benzene—observed at a higher temperature of 295 K. The stability of these polymorphs was correctly predicted, with phase II being energetically preferred by 3.7 kJ⋅mol−1 over phase I. The proposed approach gives a tremendous potential to predict stability of other molecular crystal polymorphs. [ABSTRACT FROM AUTHOR]