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del_parameter [2015/08/14 09:30]
johnsoevans created
del_parameter [2020/07/16 11:29]
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-====== 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 
- 
-The loop is performed 5 times. ​ 
- 
  

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