CCL: BSSE Counterpoise correction

 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 electronic 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 the 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 overestimated. 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.
 Now in your case, if you want to find out the correct interaction energy of
 the bio-molecular assembly AB then run CP calculation on AB to get the BSSE
 correction (say E_BSSE). So your final interaction energy should be, E_AB -
 E_A - E_B + E_BSSE. Similarly if you are interested 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 correction energy
 obtained from the counterpoise calculation performed on (AB)*.
 Hope this helps.
 On Sat, Jun 29, 2019 at 5:01 AM Lee Jones bunglinpie[*] <
 owner-chemistry]~[> wrote:
 > Sent to CCL by: "Lee  Jones" [bunglinpie|,|]
 > 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:
 > 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>
 If you think you can, you are right.