BSSE

Diffuse functions are those functions with small Gaussian exponents, thus describing the wave function far from the nucleus. It is common to add additional diffuse functions to a basis. The most frequent reason for doing this is to describe orbitals with a large spatial extent, such as the HOMO of an anion or Rydberg orbitals. Adding diffuse functions can also result in a greater tendency to develop basis set superposition error (BSSE), as described later in this chapter. 

BSSE (basis set superposition error) an error introduced when using an incomplete basis set... 

An approximate way of assessing BSSE is the Counterpoise (CP) correction." In this mediod the BSSE is estimated as the difference between monomer energies with the regular basis and the energies calculated with the full set of basis functions for the whole complex. Consider two molecules A and B, each having regular nuclear centred basis sets denoted by subscripts a and b, and the complex AB having the combined basis set... 

There are variations of this method. For example may it be argued that the full set of ghost orbitals should not be used, since some of the functions in the complex are used for describing the electrons of the other component, and only the virtual orbitals are available for artificial stabilization. However, it appears that the method of full counterpoise corection (using all basis functions as ghost orbitals) gives the best results. Note that A cp is an approximate correction, it gives an estimate of the BSSE effect, but it does not provide either an upper or lower limit. 

There have been attempts to develop methods where the BSSE is excluded explicitly in the computational expressions, an example of this is the Chemical Hamiltonian Approach (CHA)," but such methods are not yet commonly used. 

Bbschungswlnkel, m. angle of slope. bSse, a. bad, evil harmful, noxious sore, aching. — boses Wesen, epilei y. — bdses Wetter, choke damp. 

Table 12-4. Computed interaction energy (AE) and counterpoise-corrected interaction energies (AECP) of the water dimer [kcal/mol]. The BSSE is given in parentheses.

In a different context, very similar conclusions about the BSSE in density functional applications were obtained by Dargel et al., 1998. 

Paizs, B., Suhai, S., 1998, Comparative Study of BSSE Correction Methods at DFT and MP2 Levels of Theory, J. Comput. Chem., 19, 575. 

Vahron, P., Vibok, A., Mayer, I., 1993, Comparision of a Posteriori and a Priori BSSE Correction Schemes for SCF Intermolecular Energies , J. Comput. Chem., 14, 401. 

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