Contracted Gaussian basis sets for molecular calculations are derived from uncontracted (12,8) and (12,9) sets for the neutral second row atoms, Z=11–18, and for the negative ions P−, S−, and Cl−. Calculations on Na...2p63p, 2P and Mg...2p63s3p, 3P are used to derive contracted Gaussian functions to describe the 3p orbital in these atoms, necessary in molecular applications. The derived basis sets range from minimal, through double-zeta, to the largest set which has a triple-zeta basis for the 3p orbital, double-zeta for the remaining. Where necessary to avoid unacceptable energy losses in atomic wave functions expanded in the contracted Gaussians, a given uncontracted Gaussian function is used in two contracted functions. These tabulations provide a hierarchy of basis sets to be used in designing a convergent sequence of molecular computations, and to establish the reliability of the molecular properties under study.

Covalency and ionicity are orthogonal rather than antipodal concepts. We demonstrate for the case of siloxane systems [R Si-(O-SiR ) -O-SiR ] that both covalency and ionicity of the Si-O bonds impact on the basicity of the Si-O-Si linkage. The relationship between the siloxane basicity and the Si-O bond character has been under debate since previous studies have presented conflicting explanations. It has been shown with natural bond orbital methods that increased hyperconjugative interactions of LP(O)→σ*(Si-R) type, that is, increased orbital overlap and hence covalency, are responsible for the low siloxane basicity at large Si-O-Si angles. On the other hand, increased ionicity towards larger Si-O-Si angles has been revealed with real-space bonding indicators. To resolve this ostensible contradiction, we perform a complementary bonding analysis, which combines orbital-space, real-space, and bond-index considerations. We analyze the isolated disiloxane molecule H SiOSiH with varying Si-O-Si angles, and n-membered cyclic siloxane systems Si H O(CH ) . All methods from quite different realms show that both covalent and ionic interactions increase simultaneously towards larger Si-O-Si angles. In addition, we present highly accurate absolute hydrogen-bond interaction energies of the investigated siloxane molecules with water and silanol as donors. It is found that intermolecular hydrogen bonding is significant at small Si-O-Si angles and weakens as the Si-O-Si angle increases until no stable hydrogen-bond complexes are obtained beyond φ =168°, angles typically displayed by minerals or polymers. The maximum hydrogen-bond interaction energy, which is obtained at an angle of 105°, is 11.05 kJ mol for the siloxane-water complex and 18.40 kJ mol for the siloxane-silanol complex.

Constrained Hartree–Fock calculations have been performed to obtain wavefunctions that reproduce experimental X‐ray structure‐factor magnitudes for crystalline NH3 to within the limits of experimental error. Different model densities using both a single molecule and clusters of NH3 in the calculation of X‐ray structure‐factor magnitudes have been examined. The effects of the crystalline lattice on the experimental wavefunction of the NH3 unit can be reproducibly recovered. The construction of structure‐factor magnitudes based on normally distributed random perturbations of the experimental values has also been used to gauge the accuracy of integrated atomic properties obtained from the wavefunctions, the point at which the constraint procedure should be terminated, and the approximate error in the experimental values.

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Ab initio molecular orbital theory has been used to study the three lowest open shell states of ethylenedione, C 2 O 2 , 2-thiooxoethen-1-one, C 2 OS, and ethylenedithione, C 2 S 2 . To treat the singlet and triplet states in an even-handed manner, multiconfigurational self-consistent field theory (MCSCF), which included all the important configurations for a quantitative description of these states, was used as the basis of the investigation. Further correlation effects have been included using a multireference configuration interaction (MRCI) approach. Basis sets of triple-quality with d-and f-polarization functions were employed. Equilibrium geometries were obtained from density functional theory (DFT) calculations using the B3LYP exchange correlation functional and are presented for the 3 Σ g -, 1 ∆ g states of all three molecules. Harmonic frequencies are also presented for these states and were calculated at the MCSCF and DFT levels. The reported relative energies for the states were obtained from MRCI calculations. As expected from previous work, the ground states are confirmed to be of 3 Σ g -symmetry for all molecules, but it is only for the C 2 S 2 that this state lies below the energy of the dissociation products in their own ground states. For C 2 OS, though, this energy gap is small (5.7 kcal/mol). In contrast to a number of other calculations, the 1 ∆ g states for all three molecules are also shown to be minima on the MCSCF potential energy surfaces. The 1 ∆ g states are all close to the ground states (<10 kcal/mol). Consequently, with C 2 S 2 , this singlet state also lies below the ground state of the dissociated products. The relative energies of the 1 Σ g + states at the optimized geometries for the 1 ∆ g states have also been determined.