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The COVID-19 pandemic disrupted chemistry teaching practices globally as many courses were forced online, necessitating adaptation to the digital platform. The biggest impact was to the practical component of the chemistry curriculum—the so-called wet lab. Naively, it would be thought that computer-based teaching laboratories would have little problem in making the move. However, this is not the case as there are many unrecognized differences between delivering computer-based teaching in-person and virtually: software issues, technology, and classroom management. Consequently, relatively few “hands-on” computational chemistry teaching laboratories are delivered online. In this paper, we describe these issues in more detail and how they can be addressed, drawing on our experience in delivering a third-year computational chemistry course as well as remote hands-on workshops for the Virtual Winter School on Computational Chemistry and the European BIG-MAP project.

Determination of the chemical and spectroscopic natures of defects in materials such as hexagonal boron nitride (h-BN) remains a serious challenge for both experiment and theory. To establish basic needs for reliable calculations, we consider a model defect VNNBin h-BN in which a boron-for-nitrogen substitution is accompanied by a nitrogen vacancy, examining its lowest energy transition, (1)2A1← (1)2B1. This provides a relatively simple test system as open-shell and charge–transfer effects, which are difficult to model and can dominate defect spectroscopy, are believed to be small. We establish calculation convergence with respect to sample size using both cluster and 2D periodic models, convergence with respect to numerical issues such as the use of plane-wave or Gaussian basis set expansions, and convergence with respect to the treatment of electron correlation. The results strongly suggest that the poor performance of computational methods for defects of other natures arises through intrinsic methodological shortcomings.

A featurization algorithm based on functional class fingerprints has been implemented within the DeepChem machine learning framework. It is based on descriptors more appropriate for solvation, taking into account intermolecular properties, and has been used in the prediction of free energies of solvation. Tests carried out on solvents with a range of polarity from the FreeSolv and MNSol data sets have shown slightly better accuracy than the commonly used topology-based extended connectivity fingerprint algorithm for hydration free energies. However, improvement was not as significant as hoped and less clear for less polar solvents suggesting that further solvent-specific descriptors may need to be taken into consideration.

The effects of structural featurizers in the prediction of band gaps have been investigated through machine learning by application to a silver nanoparticle data set and 2254 potential light-harvesting materials with known band gaps. Elemental properties were extended with structural features via Voronoi polyhedra, allowing for neighbor effects and thus presumably giving a better representation of the extended system. However, we did not find any noticeably significant difference in the predictive performance of our model. The biggest improvement in our model was due to inclusion of band gaps calculated using density functional theory. This resulted in a model that could predict the band gaps of the 2254 light-harvesting materials in the data set with an accuracy reflected in a root-mean-square error of 0.232 eV and mean absolute error of 0.142 eV. Furthermore, the good performance of our model was transferable to the prediction of a set of 72 experimental band gaps that were independent of the training set, giving a root-mean-square error of 0.91 eV and mean absolute error of 0.76 eV.

Defect states in 2-D materials present many possible uses but both experimental and computational characterization of their spectroscopic properties is difficult. We provide and compare results from 13 DFT and ab initio computational methods for up to 25 excited states of a paradigm system, the VNCBdefect in hexagonal boron nitride (h-BN). Studied include: (i) potentially catastrophic effects for computational methods arising from the multireference nature of the closed-shell and open-shell states of the defect, which intrinsically involves broken chemical bonds, (ii) differing results from DFT and time-dependent DFT (TDDFT) calculations, (iii) comparison of cluster models to periodic-slab models of the defect, (iv) the starkly differing effects of nuclear relaxation on the various electronic states that control the widths of photoabsorption and photoemission spectra as broken bonds try to heal, (v) the effect of zero-point energy and entropy on free-energy differences, (vi) defect-localized and conduction/valence-band transition natures, and (vii) strategies needed to ensure that the lowest-energy state of a defect can be computationally identified. Averaged state-energy differences of 0.3 eV are found between CCSD(T) and MRCI energies, with thermal effects on free energies sometimes also being of this order. However, DFT-based methods can perform very poorly. Simple generalized-gradient functionals like PBE fail at the most basic level and should never be applied to defect states. Hybrid functionals like HSE06 work very well for excitations within the triplet manifold of the defect, with an accuracy equivalent to or perhaps exceeding the accuracy of the ab initio methods used. However, HSE06 underestimates triplet-state energies by on average of 0.7 eV compared to closed-shell singlet states, while open-shell singlet states are predicted to be too low in energy by 1.0 eV. This leads to misassignment of the ground state of the VNCBdefect. Long-range corrected functionals like CAM-B3LYP are shown to work much better and to represent the current entry level for DFT calculations on defects. As significant differences between cluster and periodic-slab models are also found, the widespread implementation of such functionals in periodic codes is in urgent need.

NMR is a powerful tool for obtaining information on the structural characterization and dynamics of proteins, and nucleic acids, and their complexes. The complexity of the spectra is such that elucidation through computational simulation is a much desired thing. However, the size of most structures of interest is such that they remain out of reach of accurate quantum chemical techniques. Fragmentation methods have been shown to be a viable means of reducing the cost of ab initiocalculations to enable the prediction of molecular properties of large systems to chemical accuracy. We look at the systematic molecular fragmentation by annihilation method for a model peptide system and show that this procedure reproduces the shielding constants of a full calculation at only a fraction of the cost. Discussion of the considerations needed in applying this method is discussed and comparison made with the results of the similar fragment molecular orbital and ONIOM methods.

Isomerism is a fundamental chemical concept, reflecting the fact that the arrangement of atoms in a molecular entity has a profound influence on its chemical and physical properties. Here we describe a previously unclassified fundamental form of conformational isomerism through four resolved stereoisomers of a transoid(BF)O(BF)-quinoxalinoporphyrin. These comprise two pairs of enantiomers that manifest structural relationships not describable within existing IUPAC nomenclature and terminology. They undergo thermal diastereomeric interconversion over a barrier of 104 ± 2 kJ mol−1, which we term ‘akamptisomerization’. Feasible interconversion processes between conceivable synthesis products and reaction intermediates were mapped out by density functional theory calculations, identifying bond-angle inversion (BAI) at a singly bonded atom as the reaction mechanism. We also introduce the necessary BAI stereodescriptors parvoand amplo. Based on an extended polytope formalism of molecular structure and stereoisomerization, BAI-driven akamptisomerization is shown to be the final fundamental type of conformational isomerization. Isolable compounds displaying new fundamental forms of conformational isomerism were last discovered in 1914 (atropisomerism) and 1961 (hindered pyramidal inversion). Now, a new form—referred to as akamptisomerism—has been described through four resolved stereoisomers of a transoid (BF)O(BF)-quinoxalinoporphyrin compound. The stereodescriptors parvoand amplo, necessary for their classification, have also been introduced.

This paper presents a systematic study of partitioning schemes for locally dense basis sets in the context of NMR shielding calculations. The partitionings explored were based exclusively on connectivity and utilized the basis sets from the pcS-nseries. Deviations from pcS-4 shieldings were calculated for a set of 28 organic molecules at the HF, B3LYP, and KT3 levels of theory, with the primary goal being the determination of an efficient scheme that achieves maximal deviations of 0.1 ppm for 1H and 1 ppm for 13C. Both atom based and group based divisions of basis sets were examined, with the latter providing the most promising results. It is demonstrated that for the systems studied, at least pcS-1 is required for all parts of the molecule. This, coupled with pcS-3 on the group of interest and pcS-2 on the adjacent groups, is sufficient to achieve the desired level of accuracy at a minimal computational expense. In addition, the suitability of the pcS-nbasis sets for post-SCF methods was confirmed through a comparison with other standard basis sets at the MP2 level.

Sustainable scientific software needs a strong collaboration framework to ensure continuity by passing on the tools, skills and knowledge needed to the next generation. The COVID-19 pandemic triggered the unexpected effect of accelerating the development of remote platforms and tools to open up collaborations to a wider global community. In this article we outline the elements needed for such a framework, such as education, tools and community building, and discuss the current advances in technology with a nod to the future.

Summary The concept of the Brueckner orbital is examined, following a resurgence of interest in wavefunctions constructed from them. The distinction between Self Consistent Field, Natural and Brueckner orbitals are discussed. Total electron densities are calculated for several examples, and correlation densities are studied. It is found that the Brueckner orbitals are more localised than SCF orbitals. The total electron density constructed from the Brueckner reference determinant with Brueckner orbitals gives qualitatively similar pictures as other correlated methods. Brueckner orbitals are found to show dissociation well.

The analytic coupled cluster Hessian is used to obtain quartic force fields by finite difference for the isomers of diazene. Use of Hessians as a starting point can yield accurate fields with relatively few calculations. Results with SCF and MP2 are also presented for comparison.

In the version of this Article originally published, the word ‘stereoisomerism’ was erroneously included in the label of the upper-right box of Fig. 1. The label within the box has been corrected and it now reads: “Constitutional isomerism (same formula, different connectivity)”. This has been corrected in the online version of the Article.