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1. Measurement of partial atomic charges by least-squares refinement of variable electron density crystallographic models.

   Michael J. Zdilla, Taylor M. (August 2020, American Crystallographic Association Annual Meeting 2020)

   Of interest in understanding electronic structure, bulk physical properties, enthalpies of phase changes, dipole moments, and numerous other properties of molecules, is the determination of realistic partial atomic charges on atoms. Atoms may take on partial positive or negative charges due to polar covalent bonds, coordinate covalent bonds, or due to formal charges imposed by Lewis structure constraints. Traditional crystallographic refinement treats each atom as a neutral, spherical atom however. We present a ongoing developments of a mode of crystallographic model refinement that permits refinement of electron density at individual atoms in order to arrive at partial atomic charges of atoms in a crystallographic model. Comparison to calculated partial charges (CHELPG, NBO, Mulliken) from quantum calculations (DFT, MP2) in both the gas phase and crystalline state will be presented. 

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2. Direct and Simple Experimental Crystallographic Method to Calculate Partial Charges with Atomistic Correspondence to ab-initio Methods

   Prakash, P.; Byrne, A. N.; Keller, T. M.; Zdilla, M. J. (August 2020, American Crystallographic Association National Meeting 2020)

   Derivation of partial charges in small and large scale molecular systems is important for modeling of various experimental and theoretical properties like dipole moments, auto-correlation functions, charge disparity, understanding of dispersion, benchmark of classical MD simulations and electrostatic potential energy surface mapping. A correspondence between theoretical calculations (based on single/small number of molecules) is usually established with macroscopic IR/Raman spectra or dipole moment measurements. Such comparisons are indirect and lack a fine mapping of electrostatic potential from theory to experiment. In a new approach developed as the experimental part of this work, partial charges are calculated from crystallographic model refinement. The experimental method exhibits a satisfactory correspondence with partial charges obtained using quantum chemistry calculations. Particularly, gas phase partial charges from CHELPG method and condensed phase Lowdin charges correlate well and validate this experimental method. 

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3. Metallic line defect in wide-bandgap transparent perovskite BaSnO ₃

   https://doi.org/10.1126/sciadv.abd4449  

   Yun, Hwanhui; Topsakal, Mehmet; Prakash, Abhinav; Jalan, Bharat; Jeong, Jong Seok; Birol, Turan; Mkhoyan, K. Andre (January 2021, Science Advances)

   null (Ed.)

   A line defect with metallic characteristics has been found in optically transparent BaSnO 3 perovskite thin films. The distinct atomic structure of the defect core, composed of Sn and O atoms, was visualized by atomic-resolution scanning transmission electron microscopy (STEM). When doped with La, dopants that replace Ba atoms preferentially segregate to specific crystallographic sites adjacent to the line defect. The electronic structure of the line defect probed in STEM with electron energy-loss spectroscopy was supported by ab initio theory, which indicates the presence of Fermi level–crossing electronic bands that originate from defect core atoms. These metallic line defects also act as electron sinks attracting additional negative charges in these wide-bandgap BaSnO 3 films. 

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4. Apples to apples comparison of standardized to unstandardized principal component analysis of methods that assign partial atomic charges in molecules

   https://doi.org/10.1039/d2ra06349b  

   Manz, Thomas A. (November 2022, RSC Advances)

   Articles by Cho et al. ( ChemPhysChem , 2020, 21 , 688–696) and Manz ( RSC Adv. , 2020, 10 , 44121–44148) performed unstandardized and standardized, respectively, principal component analysis (PCA) to study atomic charge assignment methods for molecular systems. Both articles used subsets of atomic charges computed by Cho et al. ; however, the data subsets employed were not strictly identical. Herein, an element by element analysis of this dataset is first performed to compare the spread of charge values across individual chemical elements and charge assignment methods. This reveals an underlying problem with the reported Becke partial atomic charges in this dataset. Due to their unphysical values, these Becke charges were not included in the subsequent PCA. Standardized and unstandardized PCA are performed across two datasets: (i) 19 charge assignment methods having a complete basis set limit and (ii) all 25 charge assignment methods (excluding Becke) for which Cho et al. computed atomic charges. The dataset contained ∼2000 molecules having a total of 29 907 atoms in materials. The following five methods (listed here in alphabetical order) showed the greatest correlation to the first principal component in standardized and unstandardized PCA: DDEC6, Hirshfeld-I, ISA, MBIS, and MBSBickelhaupt (note: MBSBickelhaupt does not appear in the 19 methods dataset). For standardized PCA, the DDEC6 method ranked first followed closely by MBIS. For unstandardized PCA, Hirshfeld-I (19 methods) or MBSBickelhaupt (25 methods) ranked first followed by DDEC6 in second place (both 19 and 25 methods). 

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5. Structure and charge analysis of a cyclic aluminium hydride: cyclo -1,5-bis-μ-dimethylamino-3,7-di-μ-hydrido-2,4,6,8-tetrakis(dimethylaluminium)

   https://doi.org/10.1107/S2053229622011391  

   Corfield, Peter W.; Schrier, Joshua (January 2023, Acta Crystallographica Section C Structural Chemistry)

   The title compound, [Al 4 (CH 3 ) 8 (C 2 H 7 N) 2 H 2 ], crystallizes as eight-membered rings with –(CH 3 ) 2 Al–(CH 3 ) 2 N–(CH 3 ) 2 Al– moieties connected by single hydride bridges. In the X-ray structure, the ring has a chair conformation, with the hydride H atoms being close to the plane through the four Al atoms. An optimized structure was also calculated by all-electron density functional theory (DFT) methods, which agrees with the X-ray structure but gives a somewhat different geometry for the hydride bridge. Charges on the individual atoms were determined by valence shell occupancy refinements using MoPro and also by DFT calculations analyzed by several different methods. All methods agree in assigning a positive charge to the Al atoms, negative charges to the C, N, and hydride H atoms, and small positive charges to the methyl H atoms. 

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