ABSTRACT
CrystalExplorer is a native cross-platform program supported on Windows, MacOS and Linux with the primary function of visualization and investigation of molecular crystal structures, especially through the decorated Hirshfeld surface and its corresponding two-dimensional fingerprint, and through the visualization of void spaces in the crystal via isosurfaces of the promolecule electron density. Over the past decade, significant changes and enhancements have been incorporated into the program, such as the capacity to accurately and quickly calculate and visualize quantitative intermolecular interactions and, perhaps most importantly, the ability to interface with the Gaussian and NWChem programs to calculate quantum-mechanical properties of molecules. The current version, CrystalExplorer21, incorporates these and other changes, and the software can be downloaded and used free of charge for academic research.
ABSTRACT
The constrained Hartree-Fock method using experimental X-ray diffraction data is extended and applied to the case of noncentrosymmetric molecular crystals. A new way to estimate the errors in derived properties as a derivative with respect to added Gaussian noise is also described. Three molecular crystals are examined: ammonia [NH(3)], urea [CO(NH(2))(2)], and alloxan [(CO)(4)(NH)(2)]. The energetic and electrical properties of these molecules in the crystalline state are presented. In all cases, an enhancement of the dipole moment is observed upon application of the experimental constraint. It is found that the phases of the structure factors are robustly determined by the constrained Hartree-Fock model, even in the presence of simulated noise. Plots of the electron density, electrostatic potential, and the electron localization function for the molecules in the crystal are displayed. In general, relative to the Hartree-Fock model, there is a depletion of charge around hydrogen atoms and lone pair regions, and a build-up of charge within the molecular framework near nuclei, directed along the bonds. The electron localization function plots reveal an increase in the pair density between vicinal hydrogen atoms.
ABSTRACT
A constrained wavefunction model has been used to extract a Hartree-Fock wavefunction for C2H2O4.4H2O from both low-angle (sin theta /lambda < 0.71 A-1, 571 reflections) and full (sin theta / lambda < 1.00 A-1, 968 reflections) experimental X-ray diffraction data for crystalline alpha-oxalic acid dihydrate (alpha-C2H2O4.2H2O) using polarized double-zeta and triple-zeta Gaussian basis sets. Properties obtained from the zero-flux partitioning of the total charge-density distribution derived from these wavefunctions, as well as from multipole refinement of the experimental data, are calculated and compared. This work represents the first calculation of integrated atomic properties derived from the fitting of Gaussian density functions to experimental X-ray diffraction data. In particular, atomic kinetic energies derived from experimental data are presented for the first time. The results obtained from the constrained (experimental) charge density show qualitatively similar properties to those obtained from conventional ab initio gas-phase calculations, though the quantitative differences are often substantial. The accuracy of integrated properties calculated using this procedure was established from the analysis of a wavefunction derived from simulated random-error diffraction data; that is, data obtained by adding normally distributed errors to the experimental structure factors. Analysis of this random-error wavefunction indicated that most topological properties are accurate to within approximately 5%, although the error is much larger for those properties that have a steep gradient in the region being evaluated [e.g. the value of V(2)rho(rb) at bond critical points] or are very small (e.g. the atomic dipole moment). Calculations of the constrained wavefunction using both the larger basis set and the complete set of experimental data yield results that agree quantitatively with the smaller calculations.
ABSTRACT
Constrained Hartree-Fock calculations have been performed to obtain wavefunctions that reproduce experimental X-ray structure-factor magnitudes for crystalline NH3 to within the limits of experimental error. Different model densities using both a single molecule and clusters of NH3 in the calculation of X-ray structure-factor magnitudes have been examined. The effects of the crystalline lattice on the experimental wavefunction of the NH3 unit can be reproducibly recovered. The construction of structure-factor magnitudes based on normally distributed random perturbations of the experimental values has also been used to gauge the accuracy of integrated atomic properties obtained from the wavefunctions, the point at which the constraint procedure should be terminated, and the approximate error in the experimental sigma(k) values.
