From Matthew.Harbowy -8 at 8- unilever.com Mon Jul 8 14:59:18 1996 Received: from gatekeeper.homecare.com for Matthew.Harbowy:~at~:unilever.com by www.ccl.net (8.7.5/950822.1) id OAA04487; Mon, 8 Jul 1996 14:11:34 -0400 (EDT) Received: from x500mhub.lo.u1388.unilever.com by gatekeeper.homecare.com; (5.65v3.2/1.1.8.2/24Jan95-0439PM) id AA04331; Mon, 8 Jul 1996 19:11:00 +0100 Received: by x500mhub.lo.u1388.unilever.com; Mon, 8 Jul 96 18:58:53 Z X400-Received: by /c=GB/admd=telemail/prmd=Unilever/; Relayed; 08 Jul 96 18:58:44 Z X400-Received: by mta MTAunilever in /c=GB/admd=telemail/prmd=Unilever/; Relayed; 08 Jul 96 18:58:44 Z X400-Mts-Identifier: [/c=GB/admd=telemail/prmd=Unilever/; 0543231E14C5403A-MTAunilever] Content-Identifier: 0543231E14C5403A Content-Return: Allowed X400-Content-Type: P2-1988 ( 22 ) Conversion: Allowed Priority: normal Disclose-Recipients: Prohibited Alternate-Recipient: Allowed X400-Originator: Matthew.Harbowy ^at^ unilever.com X400-Recipients: non-disclosure; Message-Id: <0543231E14C5403A*/c=gb/admd=telemail/prmd=unilever/o=unilever/ou=ccmail/s=Harbowy/g=Matthew/ /at\MHS> Date: 08 Jul 96 18:58:44 Z From: "Matthew Harbowy" To: "chemistry # - at - # www.ccl.net" (Return requested) (Receipt notification requested) Return-Receipt-To: "Matthew Harbowy" Subject: Wht usually goes wrong: my experience with MOPAC Seeing some messages on minimization problems with negative frequencies of late here, I thought I'd add some of my experiences. 1. Bad geometry choices: frequently, a bad choice of geometry definition, or worse, forcing an artificial symmetry either through bad z-matrix definition or through actual symmetry constraints, causes one to minimize to a hilltop or hillside while one's gradients happily descend to near zero. At least with MOPAC 6, and perhaps with other MOPAC's, gradients are determined from the z-matrix definition of coordinates, and if a particularly shallow twist is defined along the coordinates, while there is an (undefined) twist that is steep, you could spend a lot of computation hours in small changes in geometry that ignore the larger gradients present and fail to completely minimize the molecule. It would probably be worse for graphical-interfaces where you have little to no control over such failures. Ways to resolve: use a different minimizer, or even try XYZ to minimize in cartesian space. Or, set the gradient to minimize further than you currently are. Or: try a different program. Sometimes the cure is to see if you can repeat it on another machine or program. 2. failure to minimize: sometimes you think you've minimized enough, but you do something inadvertant that messes that up. Most programs are smart enough to make sure that you have not been subject to bizarre rounding errors, but sometimes it makes sense to double check everything. Bond angles near 180 degrees can be the biggest headache, and it is conceivable that a rounding error might cause large geometry changes. Ways to resolve: check the geometry after minimization, and before and after frequency calcs. Also, make sure you haven't invoked a series of keywords that cancel one another or set different minimization defaults. Another big bad example (in ab-initio, not mopac) is minimizing in one basis set, and then doing a freq calc in another. There's nothing stopping a novice in principle from minimizing in MM2, ie, in Hyperchem, then using a single point calculation to print frequencies in AM1. That's probably a big error. I realize many, if not most, of the people here are not novices. But perhaps we should, in true internet style, have a faq on such things! -matt