Double-zeta STOs

DZ double-zeta STO HF Hartree-Fock limit STO AE all electrons PP pseudopotential, this calculation. Energies are in a.u., and DZ and HF results are from Reference 4. 

The starting point to obtain a PP and basis set for sulphur was an accurate double-zeta STO atomic calculation4. A 24 GTO and 16 GTO expansion for core s and p orbitals, respectively, was used. For the valence functions, the STO combination resulting from the atomic calculation was contracted and re-expanded to 3G. The radial PP representation was then calculated and fitted to six gaussians, serving both for s and p valence electrons, although in principle the two expansions should be different. Table 3 gives the numerical details of all these functions. 

Table 7.1 Atomic orbital parameters used in extended Huckel calculations. Single zeta STO functions are used for B and C and double zeta STO functions are used for the transition metals...

Because of the complexity of the PHF function, only very small electronic systems were initially considered. As first example, the electronic energy of some four electron atomic systems was calculated using the Brillouin procedure [8]. For this purpose, a short double zeta STO basis set. Is, Is , 2s and 2s , with optimized exponents was used. The energy values obtained are given in Table 1. In the same table, the RHF energy values calculated with the same basis are gathered for comparison. It is seen that the PHF model introduces some electronic correlation in the wave-function. Because of the nature of the basis set formed by only s-type orbitals, only radial correlation is included which account for about 30% of the electronic correlation energy. 

Table 13 The optimized double-zeta STO-exponents obtained by dementi from accurate Hartree-Fock calculations on the atoms from He to Ne.

The values for single-zeta STOs, the i, 2 C and C2 values for double-zeta STOs, and the VSIP values can be taken from results of atomic electronic structure calculations using the Hartree-Fock method [49, 50]. There... 

Although the (9s5p) expansions look excellent to the eye, the orbitals are still not converged to the Hartree-Fock limit. In this limit, the ground-state energy is —37.688619 Eh, whereas the (9s5p) expansion with an energy of —37.685247 Eh is in error by 3.372 mEh- For comparison, a double-zeta STO expansion is in error by only 1.942 mEh. Of course, we may improve on our GTO... 

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