Dear Ian, If one has sufficiently high resolution data, full matrix refinement with SHELXL might be the best way to get real convergence. The installation of the program also converges faster (zero dependence!).
Best wishes, George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-22582 On Wed, 25 Mar 2009, Ian Tickle wrote: > Hi Kristof > > The trivial, and it turns out only, answer is "as many as it takes to > converge". Unfortunately for everyone carrying out optimisations, or > indeed almost any kind of computation, Alan Turing proved (1936) - > Google for "Turing halting problem" - that a general algorithm to solve > the halting problem for all possible program-input pairs cannot exist, > or to put it another way, there's no way to know in advance what a > program will do without actually running it. > > The number of iterations required will obviously depend on how far away > the starting parameters are from the optimum which clearly cannot be > known in advance. In any case, if you don't like the result that the > program produces then the answer doesn't lie in changing the number of > iterations: rather the answer lies in changing the input and starting > conditions, since an optimisation procedure is valid only if carried > through to convergence (otherwise the starting assumptions are not > satisfied), i.e. taking as many (or as few) iterations as it needs to > satisfy the stopping criteria. Of course you could change the stopping > criteria but they determine how accurate the result will be, so the > question then would be "how accurate do you want it?". > > So much for the theory, on a practical note I've noticed that for some > reason structure idealisation with Refmac requires a very large number > of iterations to converge (I have not tried it with other refinement > programs though, maybe I should), given that the stopping criteria are > that the RMSDs for bonds and angles should be essentially zero (they can > only be non-zero in the case that the restraints are inconsistent with > each other, e.g. rings don't close under the restraints), so maybe > someone can answer why that is, it has baffled me for a while. > > Cheers > > -- Ian > > > -----Original Message----- > > From: owner-ccp...@jiscmail.ac.uk [mailto:owner-ccp...@jiscmail.ac.uk] > On > > Behalf Of Kristof Van Hecke > > Sent: 25 March 2009 10:42 > > To: bulletin_ccp4 > > Subject: Structure idealisation Refmac_5.5.0072 > > > > Dear, > > > > I want to optimize a DNA-helix with the function "Structure > idealisation" > > in Refmac_5.5.00782 (CCP4_6.1.1). > > My question, is this performing just a geometry optimization (against > a > > library), or is there also an energy-optimization of some kind > > involved,..? > > > > And according to the number of cycles (default 10) used, different > > structural results are obtained, hence is there a means of estimating > the > > ideal number of cycles to use..? > > > > Many thanks > > > > Kristof Van Hecke > > > > -------------------------------------- > > Kristof Van Hecke, PhD > > Biomoleculaire Architectuur > > Celestijnenlaan 200 F > > B-3001 Heverlee (Leuven) > > Tel: +32(0)16327477 > > -------------------------------------- > > > > > > > > > > > > Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm for more > > information. > > > > > > Disclaimer > This communication is confidential and may contain privileged information > intended solely for the named addressee(s). It may not be used or disclosed > except for the purpose for which it has been sent. If you are not the > intended recipient you must not review, use, disclose, copy, distribute or > take any action in reliance upon it. If you have received this communication > in error, please notify Astex Therapeutics Ltd by emailing > i.tic...@astex-therapeutics.com and destroy all copies of the message and any > attached documents. > Astex Therapeutics Ltd monitors, controls and protects all its messaging > traffic in compliance with its corporate email policy. The Company accepts no > liability or responsibility for any onward transmission or use of emails and > attachments having left the Astex Therapeutics domain. Unless expressly > stated, opinions in this message are those of the individual sender and not > of Astex Therapeutics Ltd. The recipient should check this email and any > attachments for the presence of computer viruses. Astex Therapeutics Ltd > accepts no liability for damage caused by any virus transmitted by this > email. E-mail is susceptible to data corruption, interception, unauthorized > amendment, and tampering, Astex Therapeutics Ltd only send and receive > e-mails on the basis that the Company is not liable for any such alteration > or any consequences thereof. > Astex Therapeutics Ltd., Registered in England at 436 Cambridge Science Park, > Cambridge CB4 0QA under number 3751674 >
