title: On QM/MM and MO/MO cluster calculations of all-atom anisotropic displacement parameters for molecules in crystal structures.
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.
authors.affiliation: Institut für Anorganische Chemie der Universität Göttingen, Tammannstrasse 4, Göttingen, D-37077, Germany. bdittri@gwdg.de
authors.forename: B
authors.initials: B
authors.lastname: Dittrich
authors.forename: S
authors.initials: S
authors.lastname: Pfitzenreuter
authors.forename: C B
authors.initials: CB
authors.lastname: Hübschle
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D016880:Anisotropy; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D011789:Quantum Theory
chemical_list: D000596:Amino Acids