Hirshfeld atom like refinement with alternative electron density partitions

Abstract

Hirshfeld atom refinement is one of the most successful methods for the accurate determination of structural parameters for hydrogen atoms from X-ray diffraction data. This work introduces a generalization of the method [generalized atom refinement (GAR)], consisting of the application of various methods of partitioning electron density into atomic contributions. These were tested on three organic structures using the following partitions: Hirshfeld, iterative Hirshfeld, iterative stockholder, minimal basis iterative stockholder and Becke. The effects of partition choice were also compared with those caused by other factors such as quantum chemical methodology, basis set, representation of the crystal field and a combination of these factors. The differences between the partitions were small in terms of R factor (e.g. much smaller than for refinements with different quantum chemistry methods, i.e. Hartree–Fock and coupled cluster) and therefore no single partition was clearly the best in terms of experimental data reconstruction. In the case of structural parameters the differences between the partitions are comparable to those related to the choice of other factors. We have observed the systematic effects of the partition choice on bond lengths and ADP values of polar hydrogen atoms. The bond lengths were also systematically influenced by the choice of electron density calculation methodology. This suggests that GAR-derived structural parameters could be systematically improved by selecting an optimal combination of the partition and quantum chemistry method. The results of the refinements were compared with those of neutron diffraction experiments. This allowed a selection of the most promising partition methods for further optimization of GAR settings, namely the Hirshfeld, iterative stockholder and minimal basis iterative stockholder.

The influence of refinement strategies on the wavefunctions derived from an experiment

“…A systematic analysis of the impact of the theoretical framework on the prediction of the structural factors has already been performed. The effect of the Hamiltonian, basis set, thermal smearing and partition models Bytheway et al, 2002b;Jayatilaka & Dittrich, 2008;Chodkiewicz et al, 2020), the inclusion of an explicit (Bytheway et al, 2002a) or an implicit (Dittrich et al, 2012) crystalline environment through the model of surrounding charges and dipoles, and the use of a multi-determinant approach (Genoni, 2017) on HAR are well documented. However, the influence of the experimental model setup on the XWR results has not been scrutinized in depth.…”

QM/ELMO: A Multi-Purpose Fully Quantum Mechanical Embedding Scheme Based on Extremely Localized Molecular Orbitals

“…For instance, if we consider only three alkyl groups in the active introduced the aspherical deformation of the atomic electron densities due to chemical bonding and, among them, the Hirshfeld atom refinement is the technique that recently emerged as the most promising. [110][111][112][113][114][115] This strategy consists in performing a quantum chemistry calculation at each step of the refinement (usually at HF or DFT level, even if post-HF methods have also been recently considered 128,129 ) to provide quantum mechanically rigorous electron distributions that are afterwards partitioned into aspherical atomic contributions through the stockholder Hirshfeld partitioning technique 130,131 . The obtained aspherical atomic densities are used to compute thermally averaged structure factor amplitudes N9 '<G' 9O that, along with the experimental ones N9 *H@ 9O, give the # statistical agreement:…”

Relativistic Hirshfeld atom refinement of an organo-gold(I) compound

“…HAR offers full flexibility in the calculation of the molecular electron density in the context of choosing the method and basis set. This procedure is becoming an increasingly popular technique for structure refinement (Fugel et al, 2014;Woiń ska et al, 2016Woiń ska et al, , 2017Malaspina et al, 2019;Chodkiewicz et al, 2020;Kleemiss et al, 2021).…”