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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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