The semiempirical MO method SIN DO^ is extended to second-row atoms from sodium to chlorine. The basis set has a provision to include d orbitals. To retain rotational invariance in a d orbital set, a number of hybrid integrals has to be included that invalidate the zero differential overlap (ZDO) assumption even in a symmetrically orthogonalized basis set. The inclusion of d orbitals rendered the set-up of integral calculation of the original INDO method impractical. Instead of one subroutine for each integral, all explicitly calculated integrals (overlap, core, electronic repulsion) are now contained in a single subroutine under unifying aspects. The parametrization scheme includes pseudopotentials and adjusts the total energy under inclusion of zero point energies to experimental heats of formation of ground states. The vibrational frequencies for the calculation of zero point energies are obtained from calculated force constants and G matrix elements by a scaling procedure. The results for geometries, energies, and dipole moments are compared with MNDO data.
Small Sin and Al, clusters (n = 3-10) were studied with the semiempirical molecular orbital method (MO) method SINDO1. For each n, various structures were optimized to determine the most stable structure. To obtain good qualitative agreement with available ab initio calculations d orbitals had to be omitted from the basis set. Both silicon and aluminum tend to build three-dimensional structures rather than two-or one-dimensional structures, except for n = 3 or 4. The structure growth was studied by approaching various sites of stable structures with one or more atoms. It was found that silicon and aluminum exhibit different structure growth, and consequently, different most-stable structures. Ionization potentials, HOMO-LUMO energy differences, binding energies per atom, and average atomic valencies are presented.
The rotational invariance of the Fock equations is investigated. It is demonstrated that the inclusion of d functions requires additional hybrid integrals which do not occur in the original version of INDO. Calculations are performed with srtmol on binding energies, ionization potentials, and dipole moments to show the differences between the various levels of approximation.
SINDOl calculations are presented for ground state geometries, heats of formation, ionization potentials and dipole moments. These calculations are based on a new parametrization of SINDOl for second-row elements which features inclusion of 3d orbitals and zero point energies. The comparison shows an improvement over MNDO, especially for hypervalent sulphur compounds.
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