Double-zeta Gaussian basis sets

ORTHONORMALITY AND SPLIT-BASIS OR DOUBLE-ZETA GAUSSIAN BASIS SETS... 

Table II shows that Simons basis set (set I) reproduces the experimental values rather accurately for the and 2 ionization potentials, the errors being 0.14 and 0.22 eV, respectively. These results represent vast improvements over the Koopmans theorem values of 17.58 and 21.75 eV. However, the n state is grossly in error, being 1.11 eV different from experiment. (Simons, at first, mistakenly reported his value to be 1 eV lower than this, leading him to conclude, erroneously, that the EOM method gave accurate IPs for all three states with this basis set. ) Adding the polarization functions to Simons basis set (giving basis set II) lowers the calculation of the troublesome IP by 0.36 eV and leaves the other states relatively unaffected. The EOM results with Nesbet s basis (set III) are much better still all the IPs are within 0.34 eV of experiment. The calculations employing basis set IV indicate that the EOM Gaussian basis set calculations are of comparable accuracy to the double zeta Slater basis sets with polarization functions. ...

Gaussian basis set of so-called double-zeta quality... 

An important issue of the application of electronic structure theory to polyatomic systems is the selection of the appropriate basis set. As usual in quantum chemistry, a compromise between precision and computational cost has to be achieved. It is generally accepted that in order to obtain qualitatively correct theoretical results for valence excited states of polyatomic systems, a Gaussian basis set of at least double-zeta quality with polarization functions on all atoms (or at least on the heavy atoms) is necessary. For a correct description of Rydberg-type excited states, the basis set has to be augmented with additional diffuse Gaussian functions. Such basis sets were used in the calculations discussed below. 

In order to test such an application we have calculated the spin and charge structure factors from a theoretical wave function of the iron(III)hexaaquo ion by Newton and coworkers ( ). This wave function is of double zeta quality and assumes a frozen core. Since the distribution of the a and the B electrons over the components of the split basis set is different, the calculation goes beyond the RHF approximation. A crystal was simulated by placing the complex ion in a lOxIOxlOA cubic unit cell. Atomic scattering factors appropriate for the radial dependence of the Gaussian basis set were calculated and used in the analysis. 

Due to the shallow wells involved in some off-centre instabilities, the three parameter hybrid semiempirical B3LYP functional [246] implemented in Gaussian 98 [247] has also been used in shallow well cases. These calculations use the double zeta LANL2DZ basis, which employ Gaussian type orbitals and pseudopotentials to simulate the core electrons. Some semiempirical Self Consistent Charge Extended Hiickel (SCCEH) calculations have also been performed in order to study the effect of the removal of some orbitals from the basis set on the off-center instabilities. More details about this method can be found in [248]. 

Table 1.9 Typical double-zeta quality Gaussian basis sets from the GAUSSIAN98 database as listed in the Environmental Molecular Sciences Laboratory Library of Basis functions, referred to in the text. ...

The split-basis (46), outlined in Chapter 1, the separating of the Huzinaga sto-4g) basis set, Table 1.7, into the Gaussian, with the smallest exponent and so the most diffuse function, as one component and the other three primitives as the second component in the contraction, is a simple Gaussian equivalent of dementi s double-zeta Slater functions. It provides a good example of the double-zeta approach applied in Gaussian basis set theory. 

Ab initio SCF and Cl calculations have been performed on BH3CO, using a Gaussian basis set of double-zeta quality. The gas-phase heat of reaction for the reaction (8) was calculated as -10.98 kcal moF (SCF), or -14.56 kcal mol" (Cl... 

Double and Quadruple Zeta Contracted Gaussian Basis Sets for Hydrogen through Neon. 

X =-16.82, Xaa = 20.91, Xbb = 17.50, and Xcc = 12.06 were calculated [28] by solving the coupled Hartree-Fock equations in terms of localized MOs, each with an optimum gauge origin (see [29, 30]). The diamagnetic susceptibilities from an ab initio calculation with a contracted Gaussian basis set of double zeta quality plus polarization functions, Xaa = 43.53, Xbb = 119-40, x c = 136.90, agree quite well with the values tabulated above [2]. 

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