SWAT g [0] U [2]The SWAT option introduces one variable g and one fixed parameter U which enable diffuse solvent to be modeled by Babinet's principle (R. Langridge, D.A. Marvin, W.E. Seeds, H.R. Wilson, C.W. Hooper, M.H.F. Wilkins and L.D. Hamilton, J. Mol. Biol. 2 (1960) 38-64; H. Driessen, M.I.J. Haneef, G.W. Harris, B. Howlin, G. Khan and D.S. Moss, J. Appl. Cryst. 22 (1989) 510-516). The real part of the scattering factor for each non-hydrogen atom is modified as follows:
f(new) = f(old) - g * exp [ -8pi2*U*(sintheta/lambda)2 ]
The large value of U ensures that only the low theta f and hence Fc2 values are affected. Subtracting the term in g in this way from the occupied regions of the structure is equivalent to adding a corresponding diffuse scattering term in the (empty) solvent regions in its effect on all calculated Fc2 values except F(000) (which is calculated ignoring g). For proteins g usually refines to a value between 2 and 4; for small molecules without significant diffuse solvent regions it should refine to zero.
Since both extinction and diffraction from diffuse solvent tend to affect primarily the strong reflections at low diffraction angle, they tend to show the same symptoms in the analysis of variance, and so a combined warning message is printed. It will however be obvious from the type of structural problem which of the two should be applied. The program does not permit the simultaneous refinement of SWAT and EXTI.
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