Carlier Group Gaussian User Manual - Virginia Tech

Carlier Group Gaussian User Manual 41
/Wiberg
14. Basis Set Superposition Error: Counterpoise
Correction
A. Introduction
Basis Set Superposition Error (BSSE) is an error incurred during the calculation of a
super-molecule (i.e., dimer). Suppose two molecules, A and B join together to form AB
dimer, as shown below.
A + B AB
The “dimerization energy” is not simply ΔE=E AB -(E A +E B ). The term ΔE we would obtain
in this way would be artificially low due to this BSSE. In this example, suppose that
each fragment is calculated using 12 basis function orbitals, and the dimer AB, being
composed of one A and one B fragment is calculated using 24 basis function orbitals.
When the electronic energy of AB is calculated, all 24 basis function orbitals are
available to each A or B fragment, which means that electrons from one A fragment can
use the orbitals of the B fragment to stabilize it, and vice versa. The direct result of this
is that the energy of the dimer AB will be too low.
B. Running a Counterpoise calculation
In order to correct for the BSSE imposed when calculating clusters of molecular
fragments (e.g. dimers, solvated monomers, etc.), a counterpoise correction can be
implemented on an optimized geometry. The subject of how best to compensate for
BSSE is quite complex and counterpoise is just one possible way to estimate the BSSE.
But the counterpoise correction is defined as follows:
ΔE CP = E AB – (E A{AB} +E B{AB} )
Where E A{AB} signifies the energy of fragment A calculated using all the basis function
orbitals available to dimer AB.
You will need to get the optimized geometry from your previous geometry optimization
and put this in your input file. But you will need to be absolutely certain of your atom #
assignments to properly create the input file.
Every atom of the structure in the input file must be correctly assigned to its proper
molecular fragment. Use Molden or GaussView or Spartan and examine each atom to
ensure each atoms numeric assignment (e.g. O is atom #1 above, Li is atom #2, a C is
atom #3). Be very, very careful. A misassignment will destroy a BSSE
calculation. Make careful notes as you do this. Some people find it useful to