TY - JOUR

T1 - Estimated H-atom anisotropic displacement parameters: a comparison between different methods and with neutron diffraction results

AU - Munshi, Parthapratim

AU - Madsen, Anders Ø

AU - Spackman, Mark A

AU - Larsen, Sine

AU - Destro, Riccardo

N1 - Keywords: Alanine; Algorithms; Anisotropy; Crystallography; Glycine; Hydrogen; Models, Chemical; Neutron Diffraction; Uracil; X-Ray Diffraction; Xylitol

PY - 2008

Y1 - 2008

N2 - Anisotropic displacement parameters (ADPs) are compared for H atoms estimated using three recently described procedures, both among themselves and with neutron diffraction results. The results convincingly demonstrate that all methods are capable of giving excellent results for several benchmark systems and identify systematic discrepancies for several atom types. A revised and extended library of internal H-atom mean-square displacements is presented for use with Madsen's SHADE web server [J. Appl. Cryst. (2006), 39, 757-758; http://shade.ki.ku.dk], and the improvement over the original SHADE results is substantial, suggesting that this is now the most readily and widely applicable of the three approximate procedures. Using this new library--SHADE2--it is shown that, in line with expectations, a segmented rigid-body description of the heavy atoms yields only a small improvement in the agreement with neutron results. The SHADE2 library, now incorporated in the SHADE web server, is recommended as a routine procedure for deriving estimates of H-atom ADPs suitable for use in charge-density studies on molecular crystals, and its widespread use should reveal remaining deficiencies and perhaps overcome the inherent bias in the majority of such studies.

AB - Anisotropic displacement parameters (ADPs) are compared for H atoms estimated using three recently described procedures, both among themselves and with neutron diffraction results. The results convincingly demonstrate that all methods are capable of giving excellent results for several benchmark systems and identify systematic discrepancies for several atom types. A revised and extended library of internal H-atom mean-square displacements is presented for use with Madsen's SHADE web server [J. Appl. Cryst. (2006), 39, 757-758; http://shade.ki.ku.dk], and the improvement over the original SHADE results is substantial, suggesting that this is now the most readily and widely applicable of the three approximate procedures. Using this new library--SHADE2--it is shown that, in line with expectations, a segmented rigid-body description of the heavy atoms yields only a small improvement in the agreement with neutron results. The SHADE2 library, now incorporated in the SHADE web server, is recommended as a routine procedure for deriving estimates of H-atom ADPs suitable for use in charge-density studies on molecular crystals, and its widespread use should reveal remaining deficiencies and perhaps overcome the inherent bias in the majority of such studies.

U2 - 10.1107/S010876730801341X

DO - 10.1107/S010876730801341X

M3 - Journal article

C2 - 18560163

VL - A64

SP - 465

EP - 475

JO - Acta Crystallographica Section A: Foundations and Advances

JF - Acta Crystallographica Section A: Foundations and Advances

SN - 0108-7673

ER -