From owner-chemistry@ccl.net Mon Jan 30 21:26:01 2006
From: "Alexander Kollias ackollias[*]gmail.com" <owner-chemistry!=!server.ccl.net>
To: CCL
Subject: CCL: [Bad Mail]RE: OPT on large molecules
Message-Id: <-30714-060130211934-17396-n27gLqdisGfbGe+aPpAM5Q!=!server.ccl.net>
X-Original-From: Alexander Kollias <ackollias++gmail.com>
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Date: Mon, 30 Jan 2006 21:19:15 -0500
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Sent to CCL by: Alexander Kollias [ackollias::gmail.com]
Sam,
	There are several problems in attempting to carry out such calculations
in QMC.  The first problem concerns the calculation of forces in QMC.
This is due to the fact that the force estimator (by Hellmann-Feynman
theorem) has an infinite variance.  Although improved estimators can be
constructed, i.e. using Pulay corrections along with zero-variance
techniques, that will result in a finite variance. Unfortunately these
estimators do not provide sufficient improvement in the variance for
geometry optimization, if the geometry is far from equilibrium. Second
there is the problem of reoptimizing the MOs at each step in the
geometry optimization. Both of these problems are areas of active
research.  To date triatomics (XH2) are the largest molecules for which
the force has be calculated while orbital optimization has some greater
success. Finally, QMC calculations for large systems require hundreds of
thousands of hours.  This is because although the variance of the local
energy decreases as 1/sqrt(T) where T is computer time, the inherent
variance associated with a trial function tends to grow with the
magnitude of the total energy, i.e. the larger the total energy the
larger the initial variance and associated error bar of the local
energy.
regards,
	Sasha

On Mon, 2006-01-30 at 19:47 -0500, Abrash, Samuel sabrash/arichmond.edu
wrote:
> 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]> 
> 
>