X-ray constrained wavefunctions based on Hirshfeld atoms. II. Reproducibility of electron densities in crystals of α-oxalic acid dihydrate

Davidson, Max L.; Grabowsky, Simon; Jayatilaka, Dylan (2022). X-ray constrained wavefunctions based on Hirshfeld atoms. II. Reproducibility of electron densities in crystals of α-oxalic acid dihydrate. Acta crystallographica Section B, 78(3), pp. 397-415. International Union of Crystallography 10.1107/S2052520622004103

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The Hirshfeld atom-based X-ray constrained wavefunction fitting (HA-XCW)
procedure is tested for its reproducibility, and the information content of the
fitted wavefunction is critically assessed. Fourteen different -oxalic acid
dihydrate data sets are used for this purpose, and the first joint fitting to 12 of
these data sets is reported. There are systematic features in the electron density
obtained from all data sets which agree with higher level benchmark
calculations, but there are also many other strong systematic features which
disagree with the reference calculations, most notably those associated with the
electron density near the nuclei. To enhance reproducibility, three new protocols
are described and tested to address the halting problem of XCW fitting, namely:
an empirical power-function method, which is useful for estimating the accuracy
of the structure factor uncertainties; an asymptotic extrapolation method based
on ideas from density functional theory; and a ‘conservative method’ whereby
the smallest value of the regularization parameter is chosen from a series of data
sets, or subsets.

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