Abstract
Hirshfeld atom refinement (HAR) is one of the most effective methods for obtaining accurate structural parameters for hydrogen atoms from X-ray diffraction data. Unfortunately, it is also relatively computationally expensive, especially for larger molecules due to wavefunction calculations. Here, a fragmentation approach has been tested as a remedy for this problem. It gives an order of magnitude improvement in computation time for larger organic systems and is a few times faster for metal-organic systems at the cost of only minor differences in the calculated structural parameters when compared with the original HAR calculations. Fragmentation was also applied to polymeric and disordered systems where it provides a natural solution to problems that arise when HAR is applied. The concept of fragmentation is closely related to the transferable aspherical atom model (TAAM) and allows insight into possible ways to improve TAAM. Hybrid approaches combining fragmentation with the transfer of atomic densities between chemically similar atoms have been tested. An efficient handling of intermolecular interactions was also introduced for calculations involving fragmentation. When applied in fragHAR (a fragmentation approach for polypeptides) as a replacement for the original approach, it allowed for more efficient calculations. All of the calculations were performed with a locally modified version of Olex2 combined with a development version of discamb2tsc and ORCA. Care was taken to efficiently use the power of multicore processors by simple implementation of load-balancing, which was found to be very important for lowering computational time.
Highlights
Hirshfeld atom refinement (HAR, Hirshfield, 1977; Jayatilaka & Dittrich, 2008) is a leading method for accurate determination of hydrogen atom structural parameters from X-ray diffraction data which greatly outperforms traditional models based on spherical atomic densities (Capelli et al, 2014; Woinska et al, 2016; Malaspina et al, 2017) called the independent atom model (IAM)
In this work we examine HAR combined with fragmentation on a range of organic and metal–organic systems, in attempts to achieve a considerable reduction of computational time needed for quantum chemical calculations for these systems and preserve the accuracy of the original HAR approach
We tested an application of a fragmentation method in combination with HAR
Summary
Hirshfeld atom refinement (HAR, Hirshfield, 1977; Jayatilaka & Dittrich, 2008) is a leading method for accurate determination of hydrogen atom structural parameters from X-ray diffraction data which greatly outperforms traditional models based on spherical atomic densities (Capelli et al, 2014; Woinska et al, 2016; Malaspina et al, 2017) called the independent atom model (IAM). A method which seems to be the best suited for HAR can be applied: fragmentation (see e.g. Gordon et al, 2012; Collins & Bettens, 2015; Raghavachari & Saha, 2015; Herbert, 2019), which involves dividing a larger system into smaller ones and performing calculations on those (fragments) This type of approach (referred to as fragHAR) was used by Bergmann et al (2020) in HAR by applying a variant of fragmentation called molecular fractionation with conjugate caps (MFCC) introduced by Zhang & Zhang (2003). The main goal of fragmentation is to accelerate the quantum mechanical calculations and make them quite easy to achieve for very large systems This does not necessarily have to be the case for small- to medium-sized molecules, especially when parallel computing is used.
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