# Differences

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 del_parameter [2015/08/14 09:30]johnsoevans created del_parameter [2020/07/16 11:29] Line 1: Line 1: - ====== 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. ​ -

##### Toolbox 