Basis set dependence of localized orbitals

The effects of Gaussian basis set contraction and addition of polarization functions on H₂O localized orbitals have been studied at the experimental geometry. It is shown that the electric moments and moment features of localized orbitals are not influenced very much by basis set quality variations, as going from medium size to enlarged basis sets. The difference between bond pair and lone pair charge densities was found to be larger on approaching the Hartree-Fock limit. A minimal basis set, however, does not suitably characterize the localized charge distributions.     

Non-empirical calculations and basis set effects on the inversion geometry of the aniline molecule

The geometry of the amine group and the barrier to internal conversion in aniline have been studied by single-determinantab initio SCF calculations using several basis sets from minimal to double-zeta quality. The results obtained from different types and sizes of basis sets are discussed. Calculations performed with the two most flexible basis sets yield inversion barriers of 0.9–1.1 kcal/mole and angles of pyramidalization at the nitrogen atom of 38–39 ° which are in good agreement with the experimental results (1.3 kcal/ mole and 38 °). Orbital and overlap population analyses are performed and compared with the expected mesomeric and inductive effect. The calculated dipole moment 1.48–1.49 D also agrees with the experimental values (1.48–1.50 D).Dedicated to Professor O. E. Polansky, Mülheim/Ruhr, on the occasion of his 60th birthday.     

Conformation analysis of formic acid. Extended basis set SCF and CEPA calculations

SCF computations using extended DZP and TZP basis sets have been performed to determine the structure of syn and anti formic acid and the transition state for rotation of the OH group. Effects of electron correlation were accounted for by means of CEPA calculations which predict the anti conformer to be 5 kcal/mol and the transition state 14.7 kcal/mol higher in energy than the syn conformer, with probable error estimates of 0.7 kcal/mol and 2 kcal/mol respectively.     

A non-empirical molecular orbital study on the relative stabilities of adenine and guanine tautomers

The relative stabilities of a series of adenine and guanine tautomers have been calculated using anab initio Hartree-Fock-Roothaan SCF MO method. The calculated relative stabilities agree in general with the results of earlier semiempirical studies. According to the present study, tautomeric forms with regular Kekulé structure for the six-membered purine ring are the most stable. The amine-imine tautomerization of purine bases is not likely to be responsible for spontaneous mutations in DNA.     

SCF MO CI perturbation calculations of inner shell and valence shell vertical ionization potentials

A CI method for calculating inner and valence shell vertical ionization potentials is presented. It is based on ab initio SCF MO calculations for the neutral closedshell ground state followed by CI perturbation calculations for the ground and ion states including all spin and symmetry adapted singly and doubly excited configurations with respect to the main configurations of the state of interest. The state energy is computed by performing a CI calculation for a set of selected configurations, and then adding the contributions of the remaining configurations as estimated by second order Brillouin-Wigner perturbation theory. The use of the same set of MO's for all states together with the CI perturbation method makes the method rather rapid. The numerical results are, in spite of the limited Gaussian basis sets used, in good agreement with experiment.     

Systematicab initio calculations on one- and three-dimensional polyethylene

The electronic structure of polyethylene has been re-examined using Gaussian basis sets in anab initio solid-state method. Previous calculations were shown to possess a minor error in unit-cell coordinates and were corrected. Six basis sets were used and the results compared within the nearest-neighbour approximation. The effect on the calculation of increasing the number of interacting unit cells to five was studied. The latter produces no significant change in the C-H polarity; the overall conduction band width remains essentially the same but the Koopmans' ionization potential is displaced more towards the experimental value. An investigation was made into the effects of neighbouring strands on a central filament. We find that the Point Charge Perturbation model is a useful one in this context and yields a realistic interstrand distance. This is dominated by the hydrogen atoms. It is essentially unaffected by extension of the perturbing field to second neighbour strands.     

Band structure of nonclassical polymers

It is shown that fully conjugated alternant nonclassical polymers are comparatively stable systems as a result of the considerable delocalisation energy.The energy characteristics of the polymers are strongly determined by the electron spin distribution of the degenerate nonbonding molecular orbitals; the full spin configuration,S>0, is favoured. The spin densities depend on the electron correlation and alternate; this corresponds to a ferrimagnetic state of the polymer at 0°K.     

Configuration interaction methods for improving unrestricted Hartree-Fock spin densities

Limited Configuration Interaction wave functions based on Unrestricted Hartree-Fock natural orbitals are found to be easy to compute and to give much more satisfactory spin densities than are provided by techniques currently in use.     

Molecular orbital calculations based on linear combinations of fragment orbitals

A semiempirical MO method based on localized fragment orbitals has been developed, which is particularly suited for the construction of orbital correlation diagrams for the discussion of the electronic structure of complex molecules in terms of fragments and their interactions. The method allows for the inclusion of experimental ionization potentials and electron affinities of the fragments within the calculation of the Fock matrix elements and may thus form the basis of an interpretation of photoelectron spectra, comparable to the interpretation of UV spectra by means of the MIM method of Longuet-Higgins and Murrell. Several levels of approximation are discussed using the acrolein molecule as an example.     

Quantum mechanical studies of environmental effects on biomolecules

LCAO SCF calculations with a (7s, 3p/3s) atomic basis are performed on urea and hydrated urea in order to determine the preferred sites of water fixation using the supermolecule approach. It appears that up to five water molecules can be directly bound to urea in the first hydration shell.     

Quantum chemical studies on electrophilic addition

The geometries of the 2-aminoethyl cation and the isomeric protonated aziridine have been optimized using ab initio molecular orbital calculations employing the split-valence shell 4-31G basis set. The protonated aziridine is computed to be the more stable ion by 46.5 kcal/mole (4-31G level) and 44.9 kcal/mole (double-zeta basis set). The profile to interconversion is found to have a barrier of less than 15 kcal/mole (relative to the 2-aminoethyl cation) and this profile is compared with those computed for the similar ions XCH₂CH ₂ ⁺ where X=OH, F, SH and Cl.     

The electronic structure and one-electron properties of BH computed from SCF and CI wave functions

The SCF and CI wave functions for BH, obtained in calculations described in detail elsewhere [2], are compared through their electron distributions and electron moments.Taken in part from a Ph.D. thesis submitted to the University of Toronto in 1971.     

Testing the arbitrariness and limits of a pseudopotential technique through calculations on the series of diatoms HF,AlH, HCl,AlF, AlCl,F2, Cl2

The pseudopotential techniques present some degrees of freedom, the influence of which on molecular calculations must be tested to assess the stability and accuracy of the results. The present work uses a semi-local pseudopotential extracted from near Hartree-Fock atomic calculations; the shape of the inner part of the pseudoorbital, the analytic form of the pseudopotential are shown to have less influence than the choice of the valence basis set which must be optimized. The calculated molecular constants perfectly agree with the large basis set all-electron calculations, even for polar molecules.Equipe de Recherche Associée au CNRS No. 821.     

A test of the Hirshfeld definition of atomic charges and moments

Summary The Hirshfeld population analysis scheme which carves the molecular density into atomic density contributions is tested. This method does not require a reference to basis sets or their respective locations, but is based on a different physical and mathematical footing. The advantage of this method is that, when the molecular deformation density converges to the true solution, the computed net charges will necessarily converge. This method also allows a straightforward definition for local moments. About 36 molecules have been used to compute the conventional Mulliken and Löwdin population analyses with STO3G, 6311G** and Dunning-Hay split valence basis sets. These results have been compared to the estimates provided by the Hirshfeld model. The charges found in the Hirshfeld method are smaller than those from the other methods.     

Theory of isomer shift in hemin

Using the self-consistent charge extended Hückel procedure, the charge density difference at Fe⁵⁷ nucleus, between hemin and Fe⁺³ ion is calculated. This is combined with the recent value of the calibration constant, –0.23±0.02a ₀ ³ mm/sec to obtain an isomer shift of –0.374 mm/sec between hemin and Fe⁺³ in good agreement with the value –0.392 mm/sec derived from experimental data and the calculated value of the isomer shift of Fe⁺³ with respect to K₃FeF₆ from first principle covalency investigations in the latter compound. is composed of contributions from core and valence electrons of the same order of magnitude, with the latter being more than one-half of the former. The core contribution is composed of a number of terms of comparable magnitude and differing signs, whose significance is discussed.Supported by grant HL 15196-02 from the Heart and Lung Institute of National Institute of Health.     

SCF-CI studies on the electronic ground state of water: Potential energy hypersurface and spectroscopic constants

Large-scale Hartree-Fock self-consistent field calculations, employing extended Gaussian basis sets, and configuration interaction studies are performed to calculate the energy hypersurface of the electronic ground state of the water molecule and to investigate the accuracy requirements in view of the determination of molecular spectroscopic constants. From the calculated points on the hypersurface the theoretical equilibrium geometry, the force field through fourth order, the spectroscopic constants _i, x_ij, _i as well as the Darling-Dennison and Fermi resonance constants are evaluated. The CI surface yields an equilibrium structure for H₂O withr _e = 0.9501 Å and _e=105.33 ° (r _exp = 0.9572 Å and _exp = 104.52 °). The vibrational levels are obtained with a systematic error of about 2 percent and the rotational constants to about 1 percent compared to spectroscopic data. The relative energy maximum corresponding to the linear structure with = is calculated to be 11890cm^–1, within the error limits of the values deduced from experimental measurements.     

Application of the independent electron pair approach to the calculation of excitation energies,ionization potentials,and electron affinities of first row atoms

Excitation energies, first ionization potentials and electron affinities of first row atoms are calculated with a spin-adapted independent electron pair approximation (IEPA) combined with the direct determination of pair natural orbitals (PNOs). To enable comparison with molecular calculations Gaussian basis sets are used which are small enough to be also applicable to molecules. IEPA results for the above mentioned properties are accurate to 0.1–0.3 eV which is almost one order of magnitude better than the corresponding SCF-results. The same accuracy can be expected for molecules in which a localization of the doubly and singly occupied orbitals is possible, for instance for small hydrides. This is supported by the results of calculations on carbon hydrides.     

Transferability of some properties of localized molecular orbitals

The localized molecular orbitals of some related ten- and eighteen-electron systems have been studied. The transferability of the kinetic, self-interaction, Coulomb and exchange interaction energies on localized orbitals have been shown. The standard deviation of the kinetic and of the interaction energies (including exchange) are less than 2.5% except for lone pair orbitals of the oxygen atoms where the standard deviation is close to 4%.     

Theoretical approach of the conformations of benzylidene aniline using the PCILO method

The conformational properties of benzylidene aniline (BA) were investigated using the PCILO method. Evidence has been provided that the optimization of geometry plays a prominent part, in agreement with recent experimental work on photoelectron spectroscopy by Haselbach.