Subject: Wht usually goes wrong: my experience with MOPAC

From: "Matthew Harbowy" <[email-masked]>
Date: 08 Jul 96 18:58:44 Z

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