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del_parameter [2015/08/14 09:47]
johnsoevans
del_parameter [2015/08/14 10:14]
johnsoevans
Line 1: Line 1:
 ====== del parameter ====== ====== del parameter ======
 +The del parameter is important when searching for a minimum of the penalties as a function of an independent parameter. For example, for each independent parameter ‘x’ the minimum of the penalties P is determined using ‘del’. ‘del’ is set automatically for parameters that Topas knows about (fractional coordinates,​ temperature factors, etc).
 +
 +For user defined parameters del defaults to 1e-5 which may be too small for some parameters. An optimum value of del is one that traverses the minimum in around 5 steps, with each step increasing by 1.6. Thus a search is performed as follows:
 +
 +  Let s = x
 +  First point evaluated at P(x)
 +  s = s + 1.6 del
 +  Next point evaluated at P(x+s)
 +
 +  Do 
 +   s = s + 1.6 del
 +   If P is decreasing then 
 +    evaluate at P(x+s)
 +   ​else ​
 +    evaluate at P(x-s)
 +   endif
 +   Loop
 +   
  

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