Acta Crystallographica Section A

Foundations of Crystallography

Volume 68, Part 1 (January 2012)

research papers

Acta Cryst. (2012). A68, 110-116    [ doi:10.1107/S0108767311037974 ]

On QM/MM and MO/MO cluster calculations of all-atom anisotropic displacement parameters for molecules in crystal structures

B. Dittrich, S. Pfitzenreuter and C. B. Hübschle

Abstract: The practical aspects of ab initio calculation of anisotropic displacement parameters (ADPs) for molecules in crystal structures are investigated. Computationally efficient approaches to calculate ADPs are QM/MM or MO/MO methods, where quantum chemical calculations are split into a high-level and a low-level part. Such calculations allow geometry optimizations and subsequent frequency calculations of a central molecule in a cluster of surrounding molecules as found in the crystal lattice. The frequencies and associated displacements are then converted into ADPs. A series of such calculations were performed with different quantum chemical methods and basis sets on the three zwitterionic amino-acid structures of L-alanine, L-cysteine and L-threonine, where high-quality low-temperature X-ray data are available. To scale and compare calculated ADPs, X-ray ADPs from invariom refinement were used. The future use of calculated ADPs will include the investigation of systematic errors in experimental X-ray diffraction data. Completion of an isotropic structural model is already possible. Calculated ADPs might also make it possible to perform charge-density studies on data sets of limited resolution/coverage as obtained from weak scatterers, high-pressure measurements or to deconvolute electron density obtained from the maximum-entropy method.

Keywords: QM/MM; quantum mechanics/molecular mechanics; anisotropic displacement parameters.

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[ doi:10.1107/S0108767311037974/tn5019sup1.pdf ]
Details of invariom refinement and model compounds


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