Electric field‐derived point charges to mimic the electrostatics in molecular crystals

Abstract

Because of the way the electrostatic potential is defined in a crystal, it is not possible to determine potential-derived charges for atoms in a crystal. To overcome this limitation, we present a novel method for determining atomic charges for a molecule in a crystal based on a fit to the electric field at points on a surface around the molecule. Examples of fits to the electric field at points on a Hirshfeld surface, using crystal Hartree-Fock electron densities computed with a DZP basis set are presented for several organic molecular crystals. The field-derived charges for common functional groups are transferable, and reflect chemical functionality as well as the subtle effects of intermolecular interactions. The charges also yield an excellent approximation to the electric field surrounding a molecule in a crystal for use in cluster calculations on molecules in solids.

Anisotropic displacement parameters for H atoms using an ONIOM approach

“…Finally, perhaps the most immediate question we have is whether the point charges presently used for electronic embedding in the ONIOM calculations need to come from a crystal Hartree-Fock calculation. There is no doubt that they are appropriate for our purposes and, based on the agreement indices obtained in the fitting process used to determine the charges (Whitten, McKinnon et al, 2006), it is clear that they produce a realistic representation of the crystalline potential. However, we note that periodic ab initio calculations are not always routine, especially for larger molecules.…”

“…Atomic charges for the outer layer were included in the quantum mechanical Hamiltonian, so-called electronic embedding (Vreven et al, 2006), while the molecular mechanics layer provided the appropriate repulsion potentials to replicate the environment in the crystal. The atomic charges were determined from a least-squares fit to the electric field around the molecule generated from periodic Hartree-Fock calculations as outlined in detail elsewhere (Whitten, McKinnon et al, 2006). Previous studies have examined the influence of including point charges to model the crystal field in urea (Rousseau et al, 1998(Rousseau et al, , 1999, where charges on surrounding molecules were altered in a selfconsistent manner, such that they matched the Mulliken charges of the central molecule.…”

Novel tools for visualizing and exploring intermolecular interactions in molecular crystals

“…Hirshfeld surfaces have already been utilized by others as an alternative molecular surface for large macromolecules (Immel et al, 2001), and Dittrich et al (2002) have investigated the potential of using Hirshfeld surfaces of molecular fragments in investigations of functional-group additivity in oligopeptides. The use of the Hirshfeld surface need not be limited to molecular crystals; comparisons have been conducted between Hirshfeld surfaces of ions in binary ionic crystals Pendas et al, 2002) and interatomic surfaces extracted from Bader's theory of atoms in molecules (Bader, 1990), and we have recently revisited the use of Hirshfeld surfaces for determining molecular electric moments in crystals (Whitten et al, 2004).…”

Fast analytical evaluation of intermolecular electrostatic interaction energies using the pseudoatom representation of the electron density. III. Application to crystal structures via the Ewald and direct summation methods

“…To reduce the computational effort, an accurate quantum mechanical treatment can be applied to the 'central' molecule only, while the effects of the surrounding crystal field are approximated by the external potential which adds a perturbation term b H 0 H 0 to the Hamiltonian b H H of the 'central' molecule (Swerts et al, 2002). The external potential is often represented by a number of point charges positioned around the central molecule (Whitten et al, 2006;Bjornsson & Bü hl, 2012). These are related to a class of combined quantum mechanics and molecular mechanics (Warshel & Levitt, 1976;Singh & Kollman, 1986;Field et al, 1990;Bakowies & Thiel, 1996) and ONIOM (Chung et al, 2015) methods.…”