The DLPNO-CCSD(T) method is designed to study large molecular systems at significantly reduced cost relative to its canonical counterpart. However, the error in this approach is also size-extensive and relies on cancellation of errors for the calculation of relative energies. This work provides a direct comparison of canonical CCSD(T) and TightPNO DLPNO-CCSD(T) calculations of reaction energies and barriers of a broad range of chemical reactions. The dataset includes acidities, anion binding affinities, enolization, Diels−Alder, nucleophilic substitution, and atom transfer reactions and complements existing theoretical datasets in terms of system size as well as new reaction types (e.g., anion binding affinities and chlorine atom transfer reactions). The performance of DLPNO-CCSD(T) was further examined with respect to systematic variation of basis set and system size and amounts of nonbonded interaction present in the system. The errors in the DLPNO-CCSD(T) were found to be relatively insensitive to the choice of basis set for small systems but increase monotonically with system size. Additionally, calculations of barriers appear to be more challenging than reaction energies with errors exceeding 5 kJ mol −1 for many Diels−Alder reactions. Further tests on three realistic organic reactions reveal the impact of the DLPNO approximation in calculating absolute and relative barriers that are important for predictions such as stereoselectivity.

Dispersed atomic catalysts can achieve high catalytic efficiency and have the potential to enable chemical transformation of inert molecules like CO 2 . The effect of surface defects on photocatalytic reduction of CO 2 using supported single atom catalysts however requires clarification. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (O v ) and photoexcited electrons on supported single atom Cu catalysts and CO 2 reduction. Adsorption of Cu was strong to the TiO 2 surface, and charges of the Cu atoms were highly dependent on whether surface defects were present. Cu atoms with O v aided in the adsorption of activated bent CO 2 , which is key to CO 2 reduction. Our results also show that CO 2 dissociation (CO 2 * → CO* + O*), which is a proposed initial step of CO 2 reduction to hydrocarbon products, occurs very readily for a single Cu atom in an O v , with barriers of ∼0.19 eV. Such low barriers do not occur with Cu over a stoichiometric surface. Furthermore, the presence of a photoexcited electron leads to a substantial increase in reaction rate for Cu over a stoichiometric surface; the Cu/TiO 2 surface is largely inert in the absence of photoexcited electrons. Experimental results corroborate these theoretical calculations and show that activation of CO 2 occurs most readily for TiO 2 catalysts with dispersed Cu and O v . CO 2 photoreduction also occurs most readily for TiO 2 catalysts with dispersed Cu and O v , compared to TiO 2 or Cu over stoichiometric TiO 2 catalysts. We also modeled atomic Pt to understand how metals besides Cu may behave. We found that Pt over TiO 2 also activates CO 2 but that dissociation of CO 2 over Pt with O v does not occur as readily as for Cu with O v . Our results show that tailoring TiO 2 surfaces with defects in conjunction with specific atomic catalysts like Cu may lead to fast desirable photoreduction of CO 2 .

In this paper, the performance of ab initio composite methods, and a wide range of DFT methods is assessed for the calculation of interaction energies of thermal clusters of a solute in water.

In this work, contemporary quantum mechanical (QM) implicit solvent models (SMD, SM12, and COSMO-RS) and a molecular mechanical (MM) explicit solvent model were used to predict the aqueous free energy barrier of a simple Menschutkin reaction (NH3 + CH3Cl). Surprisingly, the explicit solvent approach performed the worst, while the implicit solvent models yielded reasonably accurate values that are in accord with available experimental data. The origin of the large error in the explicit solvent model was due to the use of a fixed set of Lennard-Jones parameters during the free energy perturbation (FEP) calculations. Further analyses indicate that M06-2X/6-31+G­(d,p) yielded solute–solvent interaction energies that are in good agreement with benchmark DLPNO-CCSD­(T)/CBS values. When end-state MM to M06-2X/6-31+G­(d,p) corrections were added using FEP, it significantly improved the accuracy of the explicit solvent MM result and demonstrated that the accuracy of these models may be systematically improved with end-state corrections based on a validated QM level of theory.

This work presents a systematic assessment of QM/QM′ and QM/MM models with respect to direct QM calculations for the tautomerization (neutral to zwitterion) reactions of amino acids (glycine, alanine, valine, aspartate, and neutral and protonated histidine) solvated in a 160 water cluster. The effect of varying QM region size and choice of embedding potentials, including fixed-charge and polarizable molecular mechanics force fields (TIP3P and EFP) and various semiempirical QM methods (PM7, GFN2-xTB, DFTBA, DFTB3, HF-3c, and PBEh-3c), on the accuracy of the models was examined. A surprising finding was that molecular mechanics force fields outperformed many of the semiempirical methods. Generally, the errors in the QM/QM′ and QM/MM models converge slowly with respect to the QM region size, requiring 50 or more waters to be included in the QM region before the error in the model falls below 1 kcal mol–1 of its pure QM result. Different QM region selection schemes were also compared, and it was found that selection based on Natural Population Analysis (NPA) atomic charges significantly reduced the error in the QM/QM′ and QM/MM models particularly if a low-quality embedding potential was used. It is envisaged that these results will be useful for the development of future hybrid QM models.