CCL: OPT on large molecules

 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
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