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del_parameter [2015/08/14 09:30]
johnsoevans created
del_parameter [2019/07/26 17:14]
johnsoevans
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 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: 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 +  ​Let s = x 
-First point evaluated at P(x) +  First point evaluated at P(x) 
-s = s + 1.6 del +  s = s + 1.6 del 
-Next point evaluated at P(x+s) +  Next point evaluated at P(x+s) 
-Do + 
 +  ​Do 
    s = s + 1.6 del    s = s + 1.6 del
    If P is decreasing then     If P is decreasing then 
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     evaluate at P(x-s)     evaluate at P(x-s)
    endif    endif
-Loop+   Loop 
 +  
 +Some built in values are: 
 + 
 +<code topas>
  
-The loop is performed ​times+cell parameter ​      %min = Max(1.5, 0.995 Val - .05);  %max = 1.005 Val + .05;         del = 0.0001 Val; 
 +cell angle           %min = Max(1.5, Val - .2);         %max = Val + .2;                del 0.01 
 +scale                 min 1e-15                                                         del = Abs(Val) 0.003 + 1e-10; 
 +fractional_coordinate min = Val-Constant(1/​Get(a)); ​     max = Val+Constant(1/​Get(a)); ​ del = Constant(.1 / Get(a));  
 +sh_Cij_prm ​           min = -Abs(Val) 2 - .1;            max =  Abs(Val) 2 + .1;        del = If(Val < 0, -0.01, .01) + .01 Val;
  
 +</​code>​
 +  ​
  

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