Clusters standard basis sets

Li et alP reported a theoretical study of the structure and interaction-induced dipole moment, mean polarizability and mean first hyperpolarizability of the NH3-HCl-(H20)n (n = 0 ) clusters. They relied on B3LYP calculations with large standard basis sets aug-cc-pVDZ, aug-cc-pVDZ-fBF (aug-cc-pVDZ augmented with suitable placed bond functions), aug-cc-pVTZ, d-aug-cc-pVDZ and t-aug-cc-pVDZ. The authors have reached important conclusion on the magnitude of electric properties. For the non-hydrated complex NH3-HCI the first hyperpolarizability, calculated with the d-aug-cc-pVDZ basis, is jS = — 3.35 e ao Eh while the respective result for NH3-HCI-H2O is 413.52 and increases to 886.41 for NH3-HC1-(H20)4. 

While in principle all of the methods discussed here are Hartree-Fock, that name is commonly reserved for specific techniques that are based on quantum-chemical approaches and involve a finite cluster of atoms. Typically one uses a standard technique such as GAUSSIAN-82 (Binkley et al., 1982). In its simplest form GAUSSIAN-82 utilizes single Slater determinants. A basis set of LCAO-MOs is used, which for computational purposes is expanded in Gaussian orbitals about each atom. Exchange and Coulomb integrals are treated exactly. In practice the quality of the atomic basis sets may be varied, in some cases even including d-type orbitals. Core states are included explicitly in these calculations. 

Usually the atoms on the local region include all electrons or a small core pseudopotential whereas the atoms in the outer region include a larger core and most often only the outermost ns electrons necessary to represent the metal conduction band are explicitly included in the quantum mechanical calculation. This is nowadays a standard procedure to model Ni, Cu, Ag and Pt surfaces. " " In addition, a mixed basis scheme is usually employed to further reduce the computational cost while attempting to preserve the quality of the cluster model ab initio calculations. In this scheme, the atoms in the outer region are treated with a rather limited, even minimal, basis set whereas atoms in the local region are treated with a more extended basis set. 

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