Subject: Re: How to calculate BSSE of a 3-molecule system?

From: uccatvm <[email-masked]>
Date: Mon, 17 May 1999 11:31:07 +0100 (BST)

Hi Yubo,

To compute the interaction energy of a trimer ABC, the energy
of each fragment has to be calculated in the trimer basis set
{ABC}. In the Gaussian program package, one can simply achieve 
that by using the trimer basis set to compute the energy of A 
while "massaging" the nuclear charge of all atoms of molecules B 
and C to zero (and the same for the energies of B and C).

When you fully optimize the geometry of your system ABC, then you 
also have to take into account the monomer deformation energies
Udef, which is the energy required to bring a molecule X from its 
equilibrium geometry (re) to the geometry it has in the complex (rx). 
The equation for computing the counterpoise-corrected interaction
energy then becomes (in curly brackets the basis set used):

De(CP) = E(ABC){ABC} - E(A){ABC} - E(B){ABC} - E(C){ABC} +
         + Udef(A) + Udef(B) + Udef(C)

With Udef(X) = E(X){X}(rx) - E(X){X}(re)

You might also want to look at a recent discussion on CCL on BSSE in 
calc. of activation energies. Kalju Kahn summarized the discussion on 
the list. 

Hope this helps,

Tanja

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     Tanja van Mourik                                                
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