CCL: [Bad Mail]RE: OPT on large molecules

 Sent to CCL by: "Abrash, Samuel" [sabrash!=!richmond.edu]
 Could you use quantum monte carlo to do the optimization?
 Sam
         -----Original Message-----
         From: Shobe, David dshobe[#]sud-chemieinc.com [mailto:owner-chemistry.:.ccl.net]
         Sent: Mon 1/30/2006 12:14 PM
         To: Abrash, Samuel
         Cc:
         Subject: 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%a%ccl.net [mailto:owner-chemistry%a%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.
 [Edited by jkl.:.ccl.net to removed winmail.dat attachment]