From owner-chemistry- at -ccl.net Fri Jun 28 17:21:00 2019
From: "Lee Jones bunglinpie[*]googlemail.com" <owner-chemistry() server.ccl.net>
To: CCL
Subject: CCL: BSSE Counterpoise correction
Message-Id: <-53775-190628080511-16762-Eh+5Z5Ma9/apn2U05cJb0A() server.ccl.net>
X-Original-From: "Lee  Jones" <bunglinpie###googlemail.com>
Date: Fri, 28 Jun 2019 08:05:10 -0400


Sent to CCL by: "Lee  Jones" [bunglinpie|,|googlemail.com]
Hi.  I'm after a little guidance regarding Basis Set Superposition Error.

I understand what BSSE is and how to perform a counterpoise correction 
using ghost atoms, but my question is a little more fundamental.

Considering a bimolecular addition reaction where you have reactants A 
and B that proceed to form a single molecule AB via a transition state 
AB*, what species do you actually perform the CP correction on?

I read the following article which contains the following passage:

https://scicomp.stackexchange.com/questions/3/what-is-counterpoise-
correction

"This correction will depend on the geometries of the reactants. When 
they are very far from one another, it will be very small: they don't 
influence one another. When they are very close, this effect will be 
small, for the same reasoning. It's the intermediate distances that have 
the largest BSSE. These are the distances at or approaching the 
transition state, which serves as the bottleneck for the reaction. If you 
are not accounting for the artificial improvement near the transition 
state, then you will get an incorrect approximation of the activation 
energy, the energy difference between this transition state and the 
separated-reactant limit."


This seems to suggest that, to a first approximation, I would only need 
to CP correct the transition state AB* and can effectively ignore BSSE 
for the reactants A and B at infinite distance and for the final product 
AB (i.e. the BSSE only has a small effect on the overall reaction 
energy/enthalpy) is this correct.


Thanks