From owner-chemistry -x- at -x- ccl.net Sun Jan 29 08:55:01 2006 From: "t.frankcombe|chem.leidenuniv.nl" To: CCL Subject: CCL: OPT on large molecules Message-Id: <-30682-060129071056-15692-+ybpW+XdPKBbkNKIEMN4BA\a/server.ccl.net> X-Original-From: t.frankcombe%%chem.leidenuniv.nl Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=ISO-8859-1 Date: Sun, 29 Jan 2006 06:54:33 -0500 MIME-Version: 1.0 Sent to CCL by: t.frankcombe^chem.leidenuniv.nl If you look at the derivation of normal mode analysis, you can see that it assumes that the forces acting on the atoms are zero. So no, you cannot get reliable frequencies from geometries that have not been optimised at the level of theory (including basis set!) that you're using to calculate your second derivatives. Having said that, I've never played around with how frequencies and normal modes change as the geometry moves away from the optimised structure. They will be wrong, but I don't really know how much by. Quoting "David M. Close closed~!~etsu.edu" : > Sent to CCL by: "David M. Close" [closed(_)etsu.edu] > CCLers: > I have a question about the necessity of optimizations. I have uncovered > an effect experimentally that involves changes in large molecules. By large > I mean that even partial optimizations with modest basis sets take several > days. The effect involves charge migration from one region of the molecule > to another region. I would like to examine this effect with computations. > After a weeks work I still don't have an answer because of the times taken to > determine the optimized structures. > I realize that the correct approach is to do calculations on the optimized > structures. For example, it is necessary to do frequency calculations on the > optimized structures using the same basis sets in both calculations. But > what if one didn't use the optimized structures? Suppose one only used a > good guess at the optimized structure? Would this cause the later > calculations to be invalid? Or might they be good enough to at least decide > if the effect of interest is present and that actual > optimizations are worthwhile? > Regards, Dave Close.> > > > ------------------------------------------------- This mail sent using the Science Webmailer at Leiden University, The Netherlands