From owner-chemistry |-at-| ccl.net Fri May  7 07:26:00 2010
From: "matt j tassell mtassell**gmail.com" <owner-chemistry##server.ccl.net>
To: CCL
Subject: CCL: Van Der Waals Complex
Message-Id: <-41808-100507065848-21965-AneSJK6lo+Tu27W4mdsDmw##server.ccl.net>
X-Original-From: "matt j tassell" <mtassell%a%gmail.com>
Date: Fri, 7 May 2010 06:58:47 -0400


Sent to CCL by: "matt j tassell" [mtassell]_[gmail.com]
Dear CCl

I am currently looking at the addition reaction between a alderhyde radical and a molecule containing a double bond. I can find the transition state with no problem but i am having great difficulty locating the van der waals complex that comprises the two reactants.

If i start at the transition state and lengthen the c-c bond that is being formed and re-optimize i find that i can only obtain the vdw complex with a small basis set (6-311G**)and b3lyp functional. I have read that i need to use diffuse functions to get an accurate description of the complex but as soon as i increase the size of the basis set either to 6-311++G** or aug-cc-pvtz (starting from either the original geometry or 6-311G** converged geometry) the forces converge but the displacements come nowhere near to converging. 

I have tried making the optimization convergence criteria very tight, change the step size and also using the gdiis algorithm, but none of these seem to make any difference. I suspect that the pes is very shallow and i am not describing it properly at the present.

I was hoping that if somebody has encountered these problems before and managed to find a minima for the vdw complex they might be able to offer a suggestion as to how to best proceed from here?

Many thanks in advance