ChemShell is a modular computational chemistry package with a particular focus on hybrid quantum mechanical/molecular mechanical (QM/MM) simulations. A core set of chemical data handling modules and scripted interfaces to a large number of quantum chemistry and molecular modeling packages underpin a flexible QM/MM scheme. ChemShell has been used in the study of small molecules, molecular crystals, biological macromolecules such as enzymes, framework materials including zeolites, ionic solids, and surfaces. We outline the range of QM/MM coupling schemes and supporting functions for system setup, geometry optimization, and transition‐state location (including those from the open‐source DL‐FIND optimization library). We discuss recently implemented features allowing a more efficient treatment of long range electrostatic interactions, X‐ray based quantum refinement of crystal structures, free energy methods, and excited‐state calculations. ChemShell has been ported to a range of parallel computers and we describe a number of options including parallel execution based on the message‐passing capabilities of the interfaced packages and task‐farming for applications in which a number of individual QM, MM, or QM/MM calculations can performed simultaneously. We exemplify each of the features by brief reference to published applications.This article is categorized under: Software > Quantum Chemistry Software > Molecular Modeling

The recently proposed CAM-B3LYP exchange–correlation energy functional, based on a partitioning of the r⁻¹ ₁₂ operator in the exchange interaction into long- and short-range components, is assessed for the determination of molecular thermochemistry, structures, and second order response properties. Rydberg and charge transfer excitation energies and static electronic polarisabilities are notably improved over the standard B3LYP functional; classical reaction barriers also improve. Ionisation potentials, bond lengths, NMR shielding constants and indirect spin–spin coupling constants are comparable with the two functionals. CAM-B3LYP atomisation energies and diatomic harmonic vibrational wavenumbers are less accurate than those of B3LYP. Future research directions are outlined.

A simple gradient correction to the local density approximation functional is proposed, which improves the structure of the exchange-correlation potential. The optimized generalized gradient approximation (GGA) functional provides uncoupled isotropic and anisotropic nuclear magnetic resonance shielding constants that are 2–3 times more accurate than those of commonly used GGAs, for a series of challenging molecules involving first- and second-row atoms; the results are competitive with those of ab initio wave function methods. A correlation is observed between the lowest occupied-virtual eigenvalue difference and the shielding accuracy. Magnetizabilities are also improved. The performance of the functional for structural and energetic predictions is investigated. These properties can be improved by relaxing the uniform electron gas condition, with no degradation in shielding quality. Atomization energies, ionization potentials, and molecular bond lengths are then comparable to those of other GGA functionals, although total energies are very poor.