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We present an efficient, open-source formulation for coupled-cluster theory through perturbative triples with domain-based local pair natural orbitals [DLPNO-CCSD(T)]. Similar to the implementation of the DLPNO-CCSD(T) method found in the ORCA package, the most expensive integral generation and contraction steps associated with the CCSD(T) method are linear-scaling. In this work, we show that the t1-transformed Hamiltonian allows for a less complex algorithm when evaluating the local CCSD(T) energy without compromising efficiency or accuracy. Our algorithm yields sub-kJ mol−1 deviations for relative energies when compared with canonical CCSD(T), with typical errors being on the order of 0.1 kcal mol−1, using our TightPNO parameters. We extensively tested and optimized our algorithm and parameters for non-covalent interactions, which have been the most difficult interaction to model for orbital (PNO)-based methods historically. To highlight the capabilities of our code, we tested it on large water clusters, as well as insulin (787 atoms). [ABSTRACT FROM AUTHOR]

A consideration of the point group symmetry of molecules is often advantageous from a computational efficiency standpoint and sometimes necessary for the correct treatment of chemical physics problems. Many modern electronic structure software packages include a treatment of symmetry, but these are sometimes incomplete or unusable outside of that program's environment. Therefore, we have developed the MolSym package for handling molecular symmetry and its associated functionalities to provide a platform for including symmetry in the implementation and development of other methods. Features include point group detection, molecule symmetrization, arbitrary generation of symmetry element sets and character tables, and symmetry adapted linear combinations of real spherical harmonic basis functions, Cartesian displacement coordinates, and internal coordinates. We present some of the advantages of using molecular symmetry as achieved by MolSym, particularly with respect to Hartree–Fock theory, and the reduction of finite difference displacements in gradient/Hessian computations. This package is designed to be easily integrated into other software development efforts and may be extended to further symmetry applications. [ABSTRACT FROM AUTHOR]

Community efforts in the computational molecular sciences (CMS) are evolving toward modular, open, and interoperable interfaces that work with existing community codes to provide more functionality and composability than could be achieved with a single program. The Quantum Chemistry Common Driver and Databases (QCDB) project provides such capability through an application programming interface (API) that facilitates interoperability across multiple quantum chemistry software packages. In tandem with the Molecular Sciences Software Institute and their Quantum Chemistry Archive ecosystem, the unique functionalities of several CMS programs are integrated, including CFOUR, GAMESS, NWChem, OpenMM, Psi4, Qcore, TeraChem, and Turbomole, to provide common computational functions, i.e., energy, gradient, and Hessian computations as well as molecular properties such as atomic charges and vibrational frequency analysis. Both standard users and power users benefit from adopting these APIs as they lower the language barrier of input styles and enable a standard layout of variables and data. These designs allow end-to-end interoperable programming of complex computations and provide best practices options by default.

Over the past half century, computational chemistry has evolved from a niche field to a ubiquitous pillar of modern chemical research. Driven by the increased demand for computational chemistry in research settings, the undergraduate curriculum has evolved alongside to ensure that students are well-equipped for modern research. Toward this end, many excellent computational chemistry exercises have been developed that aim to teach students what kinds of questions computational chemistry can answer and how to properly interpret the results. However, there has been far less attention given to the complexities of determining how reliable computational results are and how constraining computational scaling can be. We present an undergraduate lab exercise that uses ab initio methods to predict the combustion energy of methane. The exercise walks students through the process of benchmarking errors on a small system (methane), estimating the computational cost to perform the same analysis on a larger system (propane), and justifying an affordable yet accurate method for a hypothetical study of the larger system. Furthermore, students are introduced to other cost-saving measures like basis set extrapolation and additive corrections. The entire exercise is intentionally designed to require little technical knowledge of computational chemistry and to be flexibly grafted into a standard undergraduate curriculum. In order to ensure accessibility, the exercise utilizes the free open source software Psi4 (available on any operating system) and provides a detailed installation and use guide for completing this lab. This lab will provide students the understanding of how to properly judge, select, and justify different computational models where cost and accuracy compete, a highly desirable set of skills that generalize to any computational science.

Multifidelity modeling is a technique for fusing the information from two or more datasets into one model. It is particularly advantageous when one dataset contains few accurate results and the other contains many less accurate results. Within the context of modeling potential energy surfaces, the low-fidelity dataset can be made up of a large number of inexpensive energy computations that provide adequate coverage of the N-dimensional space spanned by the molecular internal coordinates. The high-fidelity dataset can provide fewer but more accurate electronic energies for the molecule in question. Here, we compare the performance of several neural network-based approaches to multifidelity modeling. We show that the four methods (dual, Δ-learning, weight transfer, and Meng–Karniadakis neural networks) outperform a traditional implementation of a neural network, given the same amount of training data. We also show that the Δ-learning approach is the most practical and tends to provide the most accurate model. [ABSTRACT FROM AUTHOR]

Lead is heavily utilised in industry, but it bears potential harm due to its toxic nature. Identifying lead is important in diagnosing and preventing lead poisoning. Hydride generation via inductively-coupled plasma mass spectrometer (HG-ICP-MS) can detect small amounts of lead with high accuracy. Here we investigate seven structures of Pb $ _2 $ 2 H $ _2^+ $ 2 + . The geometries of each structure were optimised using coupled cluster with single, double, and perturbative triple excitations [CCSD(T)] method with the cc-pwCVQZ-PP basis set for lead and the cc-pVQZ basis set for hydrogen. Likewise, harmonic vibrational frequencies were obtained using the same method. Anharmonic frequencies were determined using CCSD(T) with cc-pwCVTZ-PP and cc-pVTZ basis sets for lead and hydrogen, respectively. The focal point approach (FPA), including CCSDT and CCSDT(Q) computations, was applied to the seven structures to find the relative energies with respect to the global minimum butterfly structure. Relative to the global minimum butterfly structure, the energies for the planar transition state, monobridged, vinylidene-like, trans, cis, and linear structures are: 1.1, 18.3, 26.5, 27.8, 28.8, and 93.7 kcal mol $ ^{-1} $ − 1 , respectively. Natural bond orders and dipole moments are reported as well. The predicted vibrational frequencies should be helpful in guiding the laboratory observation of Pb $ _2 $ 2 H $ _2^+ $ 2 + . [ABSTRACT FROM AUTHOR]

Psi4 is an ab initio electronic structure program providing methods such as Hartree-Fock, density functional theory, configuration interaction, and coupled-cluster theory. The 1.1 release represents a major update meant to automate complex tasks, such as geometry optimization using complete-basis-set extrapolation or focal-point methods. Conversion of the top-level code to a Python module means that Psi4 can now be used in complex workflows alongside other Python tools. Several new features have been added with the aid of libraries providing easy access to techniques such as density fitting, Cholesky decomposition, and Laplace denominators. The build system has been completely rewritten to simplify interoperability with independent, reusable software components for quantum chemistry. Finally, a wide range of new theoretical methods and analyses have been added to the code base, including functional-group and open-shell symmetry adapted perturbation theory, density-fitted coupled cluster with frozen natural orbitals, orbital-optimized perturbation and coupled-cluster methods (e.g., OO-MP2 and OO-LCCD), density-fitted multiconfigurational self-consistent field, density cumulant functional theory, algebraic-diagrammatic construction excited states, improvements to the geometry optimizer, and the "X2C" approach to relativistic corrections, among many other improvements.

Criegee intermediates, formed from the ozonolysis of alkenes, are known to have a role in atmospheric chemistry, including the modulation of the oxidizing capacity of the troposphere. Although studies have been conducted since their discovery, the synthesis of these species in the laboratory has ushered in a new wave of investigations of these structures, both theoretically and experimentally. In some of these theoretical studies, high-order corrections for correlation energy are included to account for the mid multi-reference character found in these systems. Many of these studies include a focus on kinetics; therefore, the calculated energies should be accurate (<1 kcal/mol in error). In this research, we compute the enthalpies of formation for a small set of Criegee intermediates, including higher-order coupled cluster corrections for correlation energy up to coupled cluster with perturbative quintuple excitations. The enthalpies of formation for formaldehyde oxide, anti-acetaldehyde oxide, syn-acetaldehyde oxide, and acetone oxide are presented at 0 K as 26.5, 15.6, 12.2, and 0.1 kcal mol−1, respectively. Additionally, we do not recommend the coupled cluster with perturbative quadruple excitations [CCSDT(Q)] energy correction, as it is approximately twice as large as that of the coupled cluster with full quadruple excitations (CCSDTQ). Half of the CCSDT(Q) energy correction may be included as a reliable, cost-effective estimation of CCSDTQ energies for Criegee intermediates. [ABSTRACT FROM AUTHOR]

Thallium chemistry is experiencing unprecedented importance. Therefore, it is valuable to characterize some of the simplest thallium compounds. Stationary points along the singlet and triplet Tl 2H 2 potential energy surface have been characterized. Stationary point geometries were optimized with the CCSD(T)/aug‐cc‐pwCVQZ‐PP method. Harmonic vibrational frequencies were computed at the same level of theory while anharmonic vibrational frequencies were computed at the CCSD(T)/aug‐cc‐pwCVTZ‐PP level of theory. Final energetics were obtained with the CCSDT(Q) method. Basis sets up to augmented quintuple‐zeta cardinality (aug‐cc‐pwCV5Z‐PP) were employed to obtain energetics in order to extrapolate to the complete basis set limits using the focal point approach. Zero‐point vibrational energy corrections were appended to the extrapolated energies in order to determine relative energies at 0 K. It was found that the planar dibridged isomer lies lowest in energy while the linear structure lies highest in energy. The results were compared to other group 13 M 2H 2 (M = B, Al, Ga, In, and Tl) theoretical studies and some interesting variations are found. With respect to experiment, incompatibilities exist. [ABSTRACT FROM AUTHOR]

Five structures of Ge2H2 and Ge2H2+ are investigated in this study. Optimized geometries at the CCSD(T)/cc-pwCVQZ-PP level of theory were obtained. Focal point analyses were performed on these optimized geometries to determine relative energies using the CCSD(T) method with polarized basis sets up to quintuple-zeta. Energy corrections include full T and pertubative (Q) coupled-cluster effects plus anharmonic corrections to the zero-point vibrational energy. Relative ordering in energy from lowest to highest of the five Ge2H2+ structures is butterfly, germylidene, monobridged, trans, then linear. In neutral Ge2H2, the monobridged structure lies lower in energy than the germylidene structure. Fundamental vibrational frequencies and IR intensities were computed for the minima at the CCSD(T)/cc-pwCVTZ-PP level of theory to compare with experimental research. Partial atomic charges and natural bonding orbital analyses indicated that the positive charge of Ge2H2+ is contained in the region of the Ge–Ge bond. [ABSTRACT FROM AUTHOR]

In 2021 Olaru, Mebs, and Beckmann reported the synthesis of remarkable cationic carbene analogues $ \mathrm{PR_2^+} $ P R 2 + and $ \mathrm{AsR_2^+} $ As R 2 + . This work followed the same group's synthesis of $ \mathrm{SbR_2^+} $ Sb R 2 + and $ \mathrm{BiR_2^+} $ Bi R 2 + . To better understand these important systems, $ \mathrm{SbH_2^+} $ Sb H 2 + and $ \mathrm{BiH_2^+} $ Bi H 2 + have been studied with high level ab initio quantum mechanical methods. Geometries were optimised with the CCSDT(Q) method with the cc-pwCVTZ-PP basis set using small core pseudopotentials. Fundamental vibrational frequencies were computed to provide theoretical predictions for future synthetic studies. Relative energies with respect to $ \mathrm{Pn^+} $ P n + + $ \mathrm{H_2} $ H 2 ( $ \mathrm{Pn} $ Pn = $ \mathrm{Sb} $ Sb , $ \mathrm{Bi} $ Bi ) were determined at the CCSDT/CBS level of theory via the Focal Point Analysis method, and anharmonic zero-point vibrational energy and higher order contributions were also computed. For $ \mathrm{SbH_2^+} $ Sb H 2 + , we obtained R = 1.697 Å and $ \theta =90.8^{\circ } $ θ = 90.8 ∘ with $ \mathrm{Sb^+ + H_2 \rightarrow SbH_2^+} $ S b + + H 2 → Sb H 2 + reaction enthalpy of $ \Delta H = -18.71\,{\rm kcal}\ {\rm mol}^{-1} $ ΔH = − 18.71 kcal mol − 1 . For $ \mathrm{BiH_2^+} $ Bi H 2 + , the analogous results were R = 1.774 Å, $ \theta =89.7^{\circ } $ θ = 89.7 ∘ , and $ \Delta H = -7.64\,{\rm kcal}\,{\rm mol}^{-1} $ ΔH = − 7.64 kcal mol − 1 for $ \mathrm{Bi^+ + H_2 \rightarrow BiH_2^+} $ B i + + H 2 → Bi H 2 + . [ABSTRACT FROM AUTHOR]

Preceding even the Hartree-Fock method, molecular integrals are the very foundation upon which quantum chemical molecular modeling depends. Discussions of molecular integrals are normally found only in advanced and technical texts or articles. The objective of the present article is to provide less experienced readers, or students in a physical/computational chemistry course, a thorough understanding of molecular integrals. Through a series of detailed Handouts, the student/reader can participate in the derivation of molecular integrals, and in turn implement them in computer code. Hartree-Fock theory is discussed in enough detail to motivate the molecular integrals and address such topics as the atomic orbital basis. An introduction to the programming language of choice, Python3, is provided, tailored toward developing the essential skills necessary for implementing molecular integrals. The article is intended to be useful not only to instructors of physical/computational chemistry, but also to any reader who has independently sought a primer on this elusive subject.

The acetonyl radical (•CH2COCH3) is relevant to atmospheric and combustion chemistry due to its prevalence in many important reaction mechanisms. One such reaction mechanism is the decomposition of Criegee intermediates in the atmosphere that can produce acetonyl radical and OH. In order to understand the fate of the acetonyl radical in these environments and to create more accurate kinetics models, we have examined the reaction system of the acetonyl radical with O2 using highly reliable theoretical methods. Structures were optimized using coupled cluster theory with singles, doubles, and perturbative triples [CCSD(T)] with an atomic natural orbital (ANO0) basis set. Energetics were computed to chemical accuracy using the focal point approach involving perturbative treatment of quadruple excitations [CCSDT(Q)] and basis sets as large as cc-pV5Z. The addition of O2 to the acetonyl radical produces the acetonylperoxy radical, and multireference computations on this reaction suggest it to be barrierless. No submerged pathways were found for the unimolecular isomerization of the acetonylperoxy radical. Besides dissociation to reactants, the lowest energy pathway available for the acetonylperoxy radical is a 1-5 H shift from the methyl group to the peroxy group through a transition state that is 3.3 kcal mol−1 higher in energy than acetonyl radical + O2. The ultimate products from this pathway are the enol tautomer of the acetonyl radical along with O2. Multiple pathways that lead to OH formation are considered; however, all of these pathways are predicted to be energetically inaccessible, except at high temperatures. [ABSTRACT FROM AUTHOR]

Sulfurous acid (H2SO3) is an infamously elusive molecule. Although some theoretical papers have supposed possible roles for it in more complicated systems, it has yet to be experimentally observed. To aid experiment in detecting this molecule, we have examined the H2O + SO2 potential energy surface at the CCSDT(Q)/CBS//CCSD(T)-F12b/cc-pVTZ-F12b level of theory to resolve standing discrepancies in previous reports and predict the gas-phase vibrational spectrum for H2SO3. We find that sulfurous acid has two potentially detectable rotamers, separated by 1.1 kcal mol−1 ΔH0K with a torsional barrier of 1.6 kcal mol−1. The sulfonic acid isomer is only 6.9 kcal mol−1 above the lowest enthalpy sulfurous acid rotamer, but the barrier to form it is 57.2 kcal mol−1. Error in previous reports can be attributed to misidentified stationary points, the use of density functionals that perform poorly for this system, and, most importantly, the basis set sensitivity of sulfur. Using VPT2+K, we determine that the intense S=O stretch fundamental of each species is separated from other intense peaks by at least 25 cm−1, providing a target for identification by infrared spectroscopy. [ABSTRACT FROM AUTHOR]

The reaction between two OH radicals has been studied in the context of combustion chemistry, atmospheric chemistry, and astrochemistry. The global minimum on the triplet reaction surface is the van der Waals complex between water and O( 3 P) atom. We optimised the geometry of the system using ab initio methods up to CCSDT/aug-cc-pVQZ. We use the focal point approach to determine an accurate dissociation energy of 1.35 kcal mol − 1 for the complex dissociating into water and O( 3 P). We also predict the anharmonic vibrational frequencies of the complex in anticipation of future experiments aimed at detecting this system. [ABSTRACT FROM AUTHOR]

Vinoxy radicals are involved in numerous atmospheric and combustion mechanisms. High-level theoretical methods have recently shed new light on the reaction of the unsubstituted vinoxy radical with O2. The reactions of 1-methylvinoxy radical and 2-methylvinoxy radical with molecular oxygen have experimental high pressure limiting rate constants, k∞, 5–7 times higher than that of the vinoxy plus O2 reaction. In this work, high-level ab initio quantum chemical computations are applied to the 2-methylvinoxy radical plus O2 system, namely, the formation and isomerization of the 1-oxo-2-propylperoxy radical, the immediate product of O2 addition to the 2-methylvinoxy radical. Multireference methods were applied to the entrance channel. No barrier to O2 addition could be located, and more sophisticated treatment of dynamic electron correlation shows that the principal difference between O2 addition to the vinoxy and 2-methylvinoxy radicals is a larger steric factor for 2-methylvinoxy + O2. This is attributed to the favorable interaction between the incoming O2 molecule and the methyl group of the 2-methylvinoxy radical. Via the focal point approach, energetics for this reaction were determined, in most cases, to chemical accuracy. The coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] correlation energy and Hartree–Fock energies were independently extrapolated to the complete basis set limit. A correction for the effect of higher excitations was computed at the CCSDT(Q)/6-31G level. Corrections for the frozen-core approximation, the Born–Oppenheimer approximation, the nonrelativistic approximation, and the zero-point vibrational energy were included. From the 1-oxo-2-propylperoxy radical, dissociation to reactants is competitive with the lowest energy isomerization pathway. The lowest energy isomerization pathway ultimately forms acetaldehyde, CO, and ·OH as the final products. [ABSTRACT FROM AUTHOR]

Alkyl combustion models depend on kinetic parameters derived from reliable experimental or theoretical energetics that are often unavailable for larger species. To this end, we have performed a comprehensive investigation of the tert-butyl radical (R· in this paper) autoignition pathways. CCSD(T)/ANO0 geometries and harmonic vibrational frequencies were obtained for key stationary points for the R· + O2 and QOOH + O2 mechanisms. Relative energies were computed to chemical accuracy (±1 kcal mol-1) via extrapolation of RCCSD(T) energies to the complete basis-set limit, or usage of RCCSD(T)-F12 methods. At 0 K, the minimum energy R· +O2 pathway involves direct elimination of HO. 2 (30.3 kcal mol-1 barrier) from the tert-butyl peroxy radical (ROO·) to give isobutene. This pathway lies well below the competing QOOH-forming intramolecular hydrogen abstraction pathway (36.2 kcal mol-1 barrier) and ROO· dissociation (35.9 kcal mol-1 barrier). The most favorable decomposition channel for QOOH radicals leads to isobutene oxide (12.0 kcal mol-1 barrier) over isobutene (18.6 kcal mol-1 barrier). For the QOOH + O2 pathways, we studied the transition states and initial products along three pathways: (1) α-hydrogen abstraction (42.0 kcal mol-1 barrier), (2) γ-hydrogen abstraction (27.0 kcal mol-1 barrier), and (3) hydrogen transfer to the peroxy moiety (24.4 kcal mol-1 barrier). The barrier is an extensive modification to the previous 18.7 kcal mol-1 value andwarrants further study. However, it is still likely that the lowest energy QOOH+O2 pathway corresponds to pathway (3).We found significant spin contamination and/or multireference character in multiple stationary points, especially for transition states stemming from QOOH. Lastly, we provide evidence for an ... surface crossing at a Cs-symmetric, intramolecular hydrogen abstraction structure. [ABSTRACT FROM AUTHOR]

The n-propylperoxy radical has been described as a molecule of critical importance to studies of low temperature combustion. Ab initio methods were used to study this three-carbon alkylperoxy radical, normal propylperoxy. Reliable CCSD(T) (coupled-cluster theory, incorporating single, double, and perturbative triple)/ANO0 geometries were predicted for the molecule's five rotamers. For each rotamer, energetic predictions were made using basis sets as large as the cc-pV5Z in conjunction with coupled cluster levels of theory up to CCSDT(Q). Along with the extrapolations, corrections for relativistic effects, zero-point vibrational energies, and diagonal Born-Oppenheimer corrections were used to further refine energies. The results indicate that the lowest conformer is the gauche-gauche (GG) rotamer followed by the gauche-trans (0.12 kcal mol-1 above GG), trans-gauche (0.44 kcal mol-1), gauche'-gauche (0.47 kcal mol-1), and trans-trans (0.57 kcal mol-1). Fundamental vibrational frequencies were obtained using second-order vibrational perturbation theory. This is the first time anharmonic frequencies have been computed for this system. The most intense IR features include all but one of the C-H stretches. The O-O fundamental (1063 cm-1 for the GG structure) also has a significant IR intensity, 19.6 km mol-1. The anharmonicity effects on the potential energy surface were also used to compute vibrationally averaged rg,0K bond lengths, accounting for zero-point vibrations present within the molecule. [ABSTRACT FROM AUTHOR]