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The evaluation of exact exchange (EXX) is an important component of quantum chemical calculations performed with ab initio and hybrid density functional methods. While evaluating exact exchange is routine in molecular quantum chemical calculations performed with localized basis sets, the non-local nature of the exchange operator presents a major impediment to the efficient use of exact exchange in calculations that employ planewave basis sets. Non-orthogonal generalized Wannier functions (NGWFs) corresponding to planewave expansions of localized basis functions are an alternative form of basis set that can be used in quantum chemical calculations. The periodic nature of these functions renders them suitable for calculations of periodic systems, while the contraction of sets of planewaves into individual basis functions reduces the number of variational parameters, permitting the construction and direct diagonalization of the Fock matrix. The present study examines how NGWFs corresponding to Fourier series representations of conventional atom-centered basis sets can be used to evaluate exact exchange in periodic systems. Specifically, an approach for constructing the exchange operator with NGWFs is presented and used to perform Hartree-Fock calculations with a series of molecules in periodically repeated simulation cells. The results demonstrate that the NGWF approach is significantly faster than the EXX method, which is a standard approach for evaluating exact exchange in periodic systems. [ABSTRACT FROM AUTHOR]

First-principles molecular dynamics simulations are used to investigate the chemical behavior of bulk thioacetaldehyde (MeC(H)S) in response to changes in pressure, P. The simulations show that these molecules oligomerize in response to applied P. Oligomerization is initiated through C---S bond formation, with constrained dynamics simulations showing that the barrier to this reaction step is lowered significantly by applied P. Subsequent reactions involving the formation of additional C---S bonds or radical processes that lead to S---S and C---C bonds lengthen the oligomers. Oligomerization is terminated through proton transfer or the formation of rings. The mechanistic details of all reactions are examined. The results indicate that the P-induced reactivity of the MeC(H)S-based system differs significantly from that of analogous MeC(H)O-based systems, which have been reported previously. Comparison with the MeC(H)O study shows that replacing oxygen with sulfur significantly lowers the P required to initiate oligomerization (from 26 GPa to 5 GPa), increases the types of reactions in which systems of this type can take part, and increases the variety of products formed through these reactions. These differences can be explained in terms of the electronic structures of these systems, which may be useful for certain high P applications. [ABSTRACT FROM AUTHOR]

Electronic structure calculations representing the molecular orbitals (MOs) with contracted planewave basis functions (CPWBFs) have been reported recently. CPWBFs are Fourier‐series representations of atom‐centered basis functions. The mathematical features of CPWBFs permit the construction of matrix–vector products, FCo, involving the application of the Fock matrix, F, to the set of occupied MOs, Co, without the explicit evaluation of F. This approach offers a theoretical speed‐up of M/n over F‐based methods, where M and n are the number of basis functions and occupied MOs, respectively. The present study reports methodological advances that permit FCo‐based optimization of wavefunction formed from CPWBFs. In particular, a technique is reported for optimizing wavefunctions by combining pseudodiagonalization techniques based on an exact representation of FCo, approximate information regarding the virtual orbital energies, and direct inversion of the iterative subspace optimization schemes to guide the wavefunction to a converged solution. This method is found to speed‐up wavefunction optimizations by factors of up to ~6 − 8 over F‐based optimization methods while providing identical results. Further, the computational cost of this technique is relatively insensitive to basis set size, thus providing further benefits in calculations using large CPWBF basis sets. The results of density functional theory calculations show that this method permits the use of hybrid exchange‐correlation (XC) functionals with a small increase in effort over analogous calculations using generalized gradient approximation XC functionals. © 2019 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Mechanochemistry, the use of mechanical stresses to activate chemical reactions, has emerged as a topic of significant interest. The present study examines the use of an approximate model for the prediction of reaction barriers under mechanochemical conditions using the ring opening of 1,3-cyclohexadiene along conrotatory and disrotatory directions as a specific test case. To do this, reaction barriers are evaluated using quantum chemical methods with an external force applied between various pairs of atoms. The results show that the consequent effects on the barrier exhibit a significant dependence on the locations of the atoms used to apply the external force, and in some cases, force-induced instabilities occur that alter the fundamental nature of the reaction pathway. The ability of an approximate model based on a second-order expansion of the force-modified potential energy with respect to nuclear coordinates to reproduce this behavior is then assessed. Good agreement between the results obtained through the quantum chemical calculations and approximate model is attained when force-induced instabilities do not occur. In addition, a strategy for predicting when such instabilities occur is presented and found to yield results that are in qualitative agreement with the quantum chemical calculations. Finally, the response of the system to the external force is interpreted in terms of the parameters entering the model, which correspond to interatomic distances and stiffnesses, and possibly sheds lights on ways to design molecules that exhibit a desired chemical response to mechanochemical conditions. [ABSTRACT FROM AUTHOR]

First-principles molecular dynamics (FPMD) simulations and static quantum chemical (QC) calculations are used to evaluate the tensile strengths, σc, of interfaces consisting of (0001) surfaces of α-Al2O3 separated by small organic species. The evaluation of σc with FPMD was achieved by performing simulations in which the simulation cell was extending in a direction normal to the fracture plane until rupture of the interface occurred. The static QC calculations employed an approach which treated fracture of the interface as a competition between uniform extension of the simulation cell and crack formation at the rupture site, which is analogous to that used in the construction of universal binding energy relationships. The results showed that the static QC calculations accurately reproduced the FPMD simulations with respect to tensile strength and the cell extension at which rupture occurred, provided that the rupture site employed in the static calculations matched the site at which rupture occurred during the FPMD simulations. A simple strategy for identifying the rupture site, even in complex systems containing many potential rupture sites, is proposed. Overall, the work extends the calculation of tensile strengths with static QC methods to highly heterogeneous interfaces, thus providing a computationally efficient alternative to demanding FPMD simulations for this purpose. [ABSTRACT FROM AUTHOR]

First-principles molecular dynamics simulations are used to investigate the chemical response of acetaldehyde molecules (MeCHO) to compression and decompression between (0001) surfaces of α-alumina (Al2O3), with pressures reaching approximately 40 GPa. The results demonstrate that the MeCHO molecules are transformed into other chemical species through a range of chemical processes involving the formation of C-O and C-C bonds between MeCHO monomers as well as proton transfer. The mechanistic details of a representative set of the observed reactions are elucidated through analysis of maximally localized Wannier functions. Analysis of the changes in structure demonstrates that the main role of compression is to reduce the distances between MeCHO molecules to facilitate the formation of C-O bonds. Additional examination of the electronic structure demonstrates that the surface plays a role in facilitating proton transfer by both rendering hydrogen atoms in adsorbed MeCHO molecules more acidic and by acting as a proton acceptor. In addition, adsorption of the MeCHO molecules on the surface renders the sp2 carbon atoms in these molecules more electrophilic, which promotes the formation of C-C and C-O bonds. It is suggested that the reaction products may be beneficial in the context of wear inhibition. Comparison of the surface structure before compression and after decompression demonstrates that the aldehydes and reaction products are capable of inhibiting irreversible changes in the structure as long as there is at least a monolayer coverage of these species. As a whole, the study sheds light on the chemical behavior of the aldehydes in response to uniaxial compression in nanoscopic contacts that likely applies to other molecules containing carbonyl groups and other metal oxide surfaces. [ABSTRACT FROM AUTHOR]

First-principles molecular dynamics simulations are used to investigate the behavior of bulk acetaldehyde (MeCHO) under conditions of increasing pressure. The results demonstrate that increasing pressure causes the aldehydes to polymerize, yielding polyethers through a process involving the rapid formation of C[Single_Bond]O bonds between multiple neighboring MeCHO molecules. Attempts to induce polyether formation at different densities through the application of geometric constraints show that polymerization occurs only once a critical density of ∼1.7 g/cm3 has been reached. The results of simulations performed at several different temperatures are also consistent with a process that is induced by reaching a critical density. The origins of this effect are rationalized in terms of the structural requirements for the formation of C[Single_Bond]O bonds between multiple MeCHO molecules in rapid succession. Specifically, the collective formation of C[Single_Bond]O bonds requires the typical distance between the sp2 carbon atoms and oxygen atoms in neighboring MeCHO molecules to reach a value of approximately 2.5 Å. Radial distribution functions calculated at different densities show that this structural requirement is reached when the density is near the observed threshold. The observed reaction may be useful in the context of lubrication, with polyethers being effective lubricants and the extreme conditions experienced in sliding contacts providing the ability to reach the high densities needed to induce the reaction. In this context, the calculations indicate that polyether formation is associated with significant energy dissipation, while energy dissipation is minimal once the polyethers are formed. Furthermore, the polyethers are stable with respect to multiple compression/decompression cycles and pressures of at least 60 GPa. [ABSTRACT FROM AUTHOR]

Conventional density functional theory (DFT) fails for strongly correlated electron systems due to large intra-atomic self-interaction errors. The DFT+U method provides a means of overcoming these errors through the use of a parametrized potential that employs an exact treatment of quantum mechanical exchange interactions. The parameters that enter into this potential correspond to the spherically averaged intra-atomic Coulomb (U) and exchange (J) interactions. Recently, we developed an ab initio approach for evaluating these parameters on the basis of unrestricted Hartree–Fock (UHF) theory, which has the advantage of being free of self-interaction errors and does not require experimental input [Mosey and Carter, Phys. Rev. B 76, 155123 (2007)]. In this work, we build on that method to develop a more robust and convenient ab initio approach for evaluating U and J. The new technique employs a relationship between U and J and the Coulomb and exchange integrals evaluated using the entire set of UHF molecular orbitals (MOs) for the system. Employing the entire set of UHF MOs renders the method rotationally invariant and eliminates the difficulty in selecting unambiguously the MOs that correspond to localized states. These aspects overcome two significant deficiencies of our earlier method. The new technique is used to evaluate U and J for Cr2O3, FeO, and Fe2O3. The resulting values of U-J are close to empirical estimates of this quantity for each of these materials and are also similar to results of constrained DFT calculations. DFT+U calculations using the ab initio parameters yield results that are in good agreement with experiment. As such, this method offers a means of performing accurate and fully predictive DFT+U calculations of strongly correlated electron materials. [ABSTRACT FROM AUTHOR]

A recently suggested mechanism for the stress memory of various metal phosphates is investigated experimentally. Based on first-principles simulations [N. J. Mosey et al., Science 307, 1612 (2005)], it had been argued that atoms with flexible coordination, such as zinc or heavy-metal cations, act as network-forming agents, undergoing irreversible pressure-induced changes in bonding that lead to increased connectivity between phosphate anions. In the present study, orthophosphates of zinc and calcium were exposed to high pressures on surfaces and in diamond anvil cells. An additional set of first-principles simulations was accomplished on α-orthophosphate of zinc, which suggested that this material was already cross-linked before compression but that it nevertheless underwent a reversible coordination change under pressure in agreement with the experimental results presented here. Raman spectra indicate an irreversible, pressure-induced loss of long-range crystallinity. The pressures required to induce these changes are around 7 GPa for the zinc phosphates, while they are close to 21 GPa for the calcium phosphates. Hydrogenation of the metal phosphate lowers the threshold pressure by approximately 2–3 GPa in both cases. Moreover, α-orthophosphate of zinc could be partially amorphisized under nonisotropic pressure on copper foils. [ABSTRACT FROM AUTHOR]

Assigning effective atomic charges that properly reproduce the electrostatic fields of molecules is a crucial step in the construction of accurate interatomic potentials. We propose a new approach to calculate these charges, which as previous approaches are, is based on the idea of charge equilibration. However, we only allow charge to flow between covalently bonded neighbors by using the concept of so-called split charges. The semiempirical fit parameters in our approach do not only reflect atomic properties (electronegativity and atomic hardness) but also bond-dependent properties. The new method contains two popular but hitherto disjunct approaches as limiting cases. We apply our methodology to a set of molecules containing the elements silicon, carbon, oxygen, and hydrogen. Effective charges derived from electrostatic potential surfaces can be predicted more than twice as accurately as with previous works, at the expense of one additional fit parameter per bond type controlling the polarizability between two bonded atoms. Additional bond-type parameters can be introduced, but barely improve the results. An increase in accuracy of only 30% over existing techniques is achieved when predicting Mulliken charges. However, this could be improved with additional bond-type parameters. [ABSTRACT FROM AUTHOR]

The methanolytic cleavage of a series of O,O-dimethyl O-aryl phosphorothioates (1a-g) catalyzed by a C,N-palladacycle, (2-[N,N-dimethylamino(methyl)phenyl]-[C.sup.1],N)(pyridine)palladium(II) triflate in methanol is described along with data for the methanolytic cleavage of 1a-g. The results have provided an example where a palladacycle-promoted solvolysis reaction has displayed a break in the Bronsted plot showing at least one intermediate, while the DFT calculations have provided insights into a more complex mechanism involving two intermediates.

A new approach is described where in the axially chiral monomers are employed for the synthesis of the chiral periodic mesoporous organosilicates. These materials are shown to be widely used in various applications, including the catalysis and separation science.

The reactions that occur between metathiophosphate (MTP) molecules are identified and examined through ab initio molecular dynamics simulations and static quantum chemical calculations at the density functional level of theory. The simulations show that certain types of MTPs could react to yield phosphate chains, while others only dimerize.

The interactions between metathiophosphates (MTPs) and zinc dialkyldithiophosphates (ZnDDPs) were explored through molecular simulation. The results reveal a novel reaction leading to formation of zinc phosphate polymers and networks through the insertion of MTPs and ZnDDPs.

The density functional theory (DFT) calculations were applied for studying a set of isomerization and decomposition reactions that Zinc dialkyldithiophosphate (ZDDP) engine oil antiwear additives may take part in. The details of the reaction pathways that these systems follow to form the precursors explain the observed byproducts of antiwear film formation.

The thermal decomposition of several zinc dialkyldithiophosphate (ZDDP) antiwear additives are investigated with both finite temperature gas-phase ab initio molecular dynamics (MD) simulations and static quantum chemical calculations at the density functional (DFT) level of theory. The results show how the observed processes will affect the overall abilities of the antiwear film.

The addition of formaldehyde to silene, disilene, germene, digermene, and germasilene in an effort to provide fundamental knowledge of the mechanistic pathway for the addition of nonenolizable aldehydes and ketones to (di)metallenes, is examined. The calculations reveal that when Ge-O bond forms during the addition, a diradical pathway is not available, whereas when a Si-O bond forms during the addition, both diradical and zwitterionic pathways are found.