# Differences

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 del_parameter [2015/08/14 09:47]johnsoevans del_parameter [2015/08/14 10:14]johnsoevans Both sides previous revision Previous revision 2019/07/26 17:14 johnsoevans 2019/07/26 17:13 johnsoevans 2015/08/14 10:14 johnsoevans 2015/08/14 10:13 johnsoevans 2015/08/14 09:47 johnsoevans 2015/08/14 09:43 johnsoevans 2015/08/14 09:35 johnsoevans 2015/08/14 09:30 johnsoevans created Next revision Previous revision 2019/07/26 17:14 johnsoevans 2019/07/26 17:13 johnsoevans 2015/08/14 10:14 johnsoevans 2015/08/14 10:13 johnsoevans 2015/08/14 09:47 johnsoevans 2015/08/14 09:43 johnsoevans 2015/08/14 09:35 johnsoevans 2015/08/14 09:30 johnsoevans created Next revision Both sides next revision Line 1: Line 1: ====== 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 +