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Vanishing of the atomic form factor derivatives in non-spherical structural refinement - a key approximation scrutinized in the case of Hirshfeld atom refinement

Lookup NU author(s): Dr Laura MidgleyORCiD

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Abstract

When calculating derivatives of structure factors, there is one particular term (the derivatives of the atomic form factors) that will always be zero in the case of tabulated spherical atomic form factors. What happens if the form factors are non-spherical? The assumption that this particular term is very close to zero is generally made in non-spherical refinements (for example, implementations of Hirshfeld atom refinement or transferable aspherical atom models), unless the form factors are refinable parameters (for example multipole modelling). To evaluate this general approximation for one specific method, a numerical differentiation was implemented within the NoSpherA2 framework to calculate the derivatives of the structure factors in a Hirshfeld atom refinement directly as accurately as possible, thus bypassing the approximation altogether. Comparing wR2 factors and atomic parameters, along with their uncertainties from the approximate and numerically differentiating refinements, it turns out that the impact of this approximation on the final crystallographic model is indeed negligible.

Publication metadata

Author(s): Midgley L, Bourhis L, Dolomanov O, Grabowsky S, Kleemiss F, Puschmann H, Peyerimhoff N

Publication type: Article

Publication status: Published

Journal: Acta Crystallographica Section A: Foundations and Advances

Year: 2021

Volume: 77

Pages: 519-533

Print publication date: 22/11/2021

Acceptance date: 02/09/2021

ISSN (electronic): 2053-2733

Publisher: International Union of Crystallography

URL: https://doi.org/10.1107/S2053273321009086

DOI: 10.1107/S2053273321009086

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