From TILBROOK %-% at %-% chem.surrey.ac.uk Tue Mar 7 10:29:43 1995 Received: from mail-server.surrey.ac.uk for TILBROOK- at -chem.surrey.ac.uk by www.ccl.net (8.6.10/930601.1506) id JAA08083; Tue, 7 Mar 1995 09:57:38 -0500 Received: from central.surrey.ac.uk by mail-server.surrey.ac.uk with SMTP (PP); Tue, 7 Mar 1995 14:57:00 +0000 Received: from chemcharon.chem.surrey.ac.uk by central.surrey.ac.uk with SMTP (1.37.109.8/16.2) id AA27454; Tue, 7 Mar 1995 14:56:26 GMT Received: From CHEMISTRY-1/WORKQUEUE by chemcharon.chem.surrey.ac.uk via Charon-4.0-VROOM with IPX id 100.950307145648.736; 07 Mar 95 14:59:04 +500 Message-Id: To: CHEMISTRY ^at^ ccl.net From: David Organization: Chemistry Department, Uni of Surrey Date: Tue, 7 Mar 1995 14:56:42 GMT Subject: Modelling IR Vibrational Modes. Priority: normal X-Mailer: WinPMail v1.0 (R1) Dear CCL Sometime ago I posted a question regarding the simulation of IR vibrational modes. Asking for the pro's and con's of various methods. I now have access to both semi-empirical methods (MOPAC) and molecular mechanics (in the form of Cerius2 by MSI). What I would be very interested to hear from anyone invloved in similar work. I have a number of questions to pose you. 1.If you are doing MD work what kind of time scale and duration do you have to run on to get reasonable results? I am presently using a time slice of 0.0001 ps and doing of the order of 100 000 iterations to provide a trajectory file to pass onto the VAC code. 2. What kind of scaling do you find is necessary to give good values for vibrational frequencies? The results I have obtained using UFF with the cross terms implemented by MSI seem to be okay but for the most part are off by a factor of approximately 0.9 (although some C-H bands seem to be quite close to the experimental values without scaling). In the vibrational visualisation module for MOPAC under Cerius2 v1.6 uses and standard scaling for all frequencies calculated by the FORCE option under MOPAC of 0.893 (quite similar to the discrepancy observed with the UFF calculations) 3. Can anybody tell me why this 'magic number' 0.9? Is there any theoretical basis for it's use or is it simply a fudge factor that seems to work? Any help you can give me with these questions or any comments you wish to make about the subject of IR simulation in general will be gratefully received and summarised to the list in due course. Thanks in advance and Kindest Regards David Tilbrook ********************************************************** David Tilbrook * Tel: 44 1483 300 800 x 9591 Polymer Group, * x 2617 Chemistry Department, * University of Surrey, * Fax: 44 1483 259 514 Guildford, Surrey, GU2 5XH * U.K. * **********************************************************