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International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 25.1, p. 690
Section 25.1.6.6. SQUASH
a
Biomolecular Crystallography Laboratory, CABM & Rutgers University, 679 Hoes Lane, Piscataway, NJ 08854-5638, USA, and Institute of Biochemistry and Cell Biology, Chinese Academy of Sciences, Yue-Yang Road, Shanghai 200 031, People's Republic of China, and bBiomolecular Crystallography Laboratory, CABM & Rutgers University, 679 Hoes Lane, Piscataway, NJ 08854-5638, USA |
The SQUASH program (Zhang & Main, 1990a![[link]](/graphics/greenarr.gif)
,b) provides a tool for phase refinement and extension of macromolecular structures. The starting point is a set of native structure factors to some resolution, and estimated phases and figures of merit for some subset of the phases. The result is a set of improved phases and figures of merit for the whole data set. The program combines Sayre's equation with density modification by histogram matching, solvent flattening and noncrystallographic symmetry averaging. The real-space formulation enables any electron-density constraint to be applied easily, e.g. solvent flattening with (eventually) known regions of density. The least-squares solution of a large system of nonlinear equations is achieved by Newton–Raphson iteration that converts the system of nonlinear equations into linear ones. The system of linear equations is solved by the conjugate-gradient method using FFTs.
Location: http://www.chem.ac.ru/Chemistry/Soft/SQUASH.en.html . Operating system: UNIX. Type: binary. Distribution: commercial.
References
Zhang, K. Y. J. & Main, P. (1990a). Histogram matching as a new density modification technique for phase refinement and extension of protein molecules. Acta Cryst. A46, 41–46.Google Scholar
Zhang, K. Y. J. & Main, P. (1990b). The use of Sayre's equation with solvent flattening and histogram matching for phase extension and refinement of protein structures. Acta Cryst. A46, 377–381.Google Scholar
