From owner-chemistry@ccl.net Mon Jan 30 11:39:01 2006 From: "Shobe, David dshobe[#]sud-chemieinc.com" To: CCL Subject: CCL: OPT on large molecules Message-Id: <-30701-060130101354-24008-sv9uuLmQtSBes8RvkxI0MA_._server.ccl.net> X-Original-From: "Shobe, David" Content-class: urn:content-classes:message Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset="iso-8859-1" Date: Mon, 30 Jan 2006 16:13:37 +0100 MIME-Version: 1.0 Sent to CCL by: "Shobe, David" [dshobe.],[.sud-chemieinc.com] We still haven't answered the main question: will the lack of optimization significantly effect the intramolecular charge transfer? I'll assume here that the effect in question does not involve vibrational frequencies. Even if the use of an arbitrary geometry doesn't materially change the results of the calculation, I would imagine you'd have some trouble convincing peer reviewers of that. But there are some options (besides just being patient :-). 1. You could use molecualr mechanics to do the optimization 2. You could use an X-ray diffraction or other published structure, if one exists. 3. You could repeat the calculation at several geometries to demonstrate that, indeed, the calculated results do not have a significant dependence on molecular geometry. The idea of checking whether the effect occurs at all, using an arbitrary but reasonable geometry, might not be a bad one. After this investigational calculation, you could then decide whetehr it's worthwhile to redo the calculation with a proper geometry (and with all the other i's dotted and t's crossed, as it were). --David Shobe, Ph.D., M.L.S. Süd-Chemie, Inc. phone (502) 634-7409 fax (502) 634-7724 Don't bother flaming me: I'm behind a firewall. -----Original Message----- > From: owner-chemistry],[ccl.net [mailto:owner-chemistry],[ccl.net] Sent: Saturday, January 28, 2006 3:48 PM To: Shobe, David Subject: CCL: OPT on large molecules 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.http://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp://www.ccl.net/chemistry/sub_unsub.shtmlhttp://www.ccl.net/spammers.txtThis e-mail message may contain confidential and / or privileged information. If you are not an addressee or otherwise authorized to receive this message, you should not use, copy, disclose or take any action based on this e-mail or any information contained in the message. If you have received this material in error, please advise the sender immediately by reply e-mail and delete this message. Thank you.