Alternative Counterpoise Scheme

A rigorous implementation of the idea that the ghost s occupied orbitals should not be available in the monomer calculation was proposed by Daudey et In their scheme, which one may call virtuals-only CP, the BSSE for monomer A is obtained as 

In the full CP method (Fig. la) the dimer and monomer Cl calculations are performed in the same manner. The occupied and virtual orbitals for each system are determined in an SCF calculation employing the complete basis set. These orbitals are subsequently employed in the Cl calculation. 

In the virtuals-only scheme (Fig. lb) the monomer calculation is performed in the basis of the monomer augmented by the virtual orbitals of the ghost, which are orthogonal to the (absent) occupied orbitals of the ghost. Note that the monomer s own virtuals are not orthogonalized to these (absent) ghost occupied orbitals. The dimer calculation may be either identical to the one of the full CP scheme above or, more in the Boys-Bernardi spirit of using identical function spaces in both dimer and monomer calculations, it may 

---

There are alternatives to the most commonly used Boys-Bemardi counterpoise scheme. One approach that shows promise, for example, is a chemical Hamiltonian approach (CHA), pioneered by Mayer " ", which attempts to isolate the superposition error directly in the Hamiltonian operator. The Schrodinger equation that is solved is hence a modified one, which yields a wave function that is hopefully free of superposition error. In the case of (HF)2, it was found that this approach mimics rather closely the results of the standard counterpoise scheme for a series of small to moderate sized basis sets ". Later calculations " extended these tests to other small H-bonded systems as well, again limiting their testing to basis sets no larger than 6-31G. A recent test " has extended the method s use-... 

Alternative counterpoise correction schemes were proposed by Olivares et and by Dykstra et Neither of these schemes produces the exact correction in the hypothetical case of a complete dimer basis, as sketched above, and so they must be rejected. 

---