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del_parameter [2015/08/14 09:47]
johnsoevans
del_parameter [2020/07/16 11:29] (current)
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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
 + 
 +Some built in values are:
 +
 +<code topas>
 +
 +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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