Hi,

BSSE comes into picture = > when you want to calculate the interaction energy of a molecular > assembly (= > say XY). Interaction energy of a molecular assembly is defined as electroni= > c energy of the complete assembly XY (E_XY) minus the sum of the electronic= > energies of individual monomer (E_X + E_Y). The problem is, to > construct t= > he wave function for XY we use more number of basis set functions than for = > X or Y. Therefore, the energy difference (E_XY - E_X -E_Y) gets overestimat= > ed. All the three energies should be calculated using same number > number of= > basis set functions and that is taken care of by the counterpoise method.<= > /div>

Now in your case, if you want to find out the corr= > ect interaction energy of the bio-molecular assembly AB then run CP calcula= > tion on AB to get the BSSE correction (say E_BSSE). So your final interacti= > on energy should be, E_AB - E_A - E_B + E_BSSE. Similarly if you are intere= > sted to find out how stable your intermediate (AB)* is, then calculate its = > interaction energy as, E_(AB)* - E_A - E_B + E*_BSSE. Here E*_BSSE is > the c= > orrection energy obtained from the counterpoise calculation performed > on (A= > B)*.

Hope this helps.

Tha= > nks,
Antarip

class=3D"gmail_quote">
ir=3D"ltr" class=3D"gmail_attr">On Sat, Jun 29, 2019 at 5:01 AM Lee > Jones b= > unglinpie[*]googlemail.com > < f=3D"mailto:owner-chemistry]~[ccl.net">owner-chemistry]~[ccl.net&g > t; wrote:= >
0px 0.8= > ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
> Sent to CCL by: "Lee=C2=A0 Jones" [bunglinpie|,| /googlemail.com" rel=3D"noreferrer" > target=3D"_blank">googlemail.com] r> > Hi.=C2=A0 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:

href=3D"https://scicomp.stackexchange.com/questions/3/what-is- counterpoi= > se-" rel=3D"noreferrer" target=3D"_blank">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 <= > br> > influence one another. When they are very close, this effect will be >
small, for the same reasoning. It's the intermediate > distances that hav= > e
> the largest BSSE. These are the distances at or approaching the
> transition state, which serves as the bottleneck for the reaction. If > you <= > br> > 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 r> > AB (i.e. the BSSE only has a small effect on the overall reaction
> energy/enthalpy) is this correct.
>
>
> Thanks
>
>
>
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