The overall standard uncertainty (estimated standard deviation) of the positional parameters based on a maximum likelihood residual. The overall standard uncertainty (sigma_X)² gives an idea of the uncertainty in the position of averagely defined atoms (atoms with B values equal to average B value) 3 N_a (sigma_X)² = --------------------------------------------------------- 8 pi² sum_i {[1/Sigma - (E_o)² (1-m²)](SUM_AS)s²} N_a = number of atoms E_o = normalized structure factors m = figure of merit of phases of reflections included in the summation s = reciprocal-space vector SUM_AS = (sigma_A)²/Sigma² Sigma = (sigma_{E;exp})² + epsilon [1-(sigma_A)²] sigma_{E;exp} = experimental uncertainties of normalized structure factors sigma_A = <cos 2 pi s delta_x> SQRT(Sigma_P/Sigma_N) estimated using maximum likelihood Sigma_P = sum_{{atoms in model}} f² Sigma_N = sum_{{atoms in crystal}} f² f = atom form factor delta_x = expected error epsilon = multiplicity of diffracting plane summation is over all reflections included in refinement Ref: (sigma_A estimation) "Refinement of macromolecular structures by the maximum-likelihood method", Murshudov, G. N., Vagin, A. A. & Dodson, E. J. (1997). Acta Cryst. D53, 240-255. (SU ML estimation) Murshudov, G. N. & Dodson, E. J. (1997). Simplified error estimation a la Cruickshank in macromolecular crystallography. CCP4 Newsletter on Protein Crystallography, No. 33, January 1997, pp. 31-39. http://www.ccp4.ac.uk/newsletters/newsletter33/murshudov.html