del_parameter
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— | del_parameter [2022/11/03 15:08] (current) – external edit 127.0.0.1 | ||
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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, | ||
+ | |||
+ | 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) | ||
+ | | ||
+ | evaluate at P(x-s) | ||
+ | endif | ||
+ | Loop | ||
+ | |||
+ | Some built in values are: | ||
+ | |||
+ | <code topas> | ||
+ | |||
+ | cell parameter | ||
+ | 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/ | ||
+ | sh_Cij_prm | ||
+ | |||
+ | </ | ||
+ | | ||