# Ab initio

Figure Al.3.24. Band structure of LiF from ab initio pseudopotentials [39], |

Figure Al.3.26. Reflectivity of LiF from ab initio pseudopotentials. (Courtesy of E L Shirley, see [39] and references therein. |

Figure Al.3.27. Energy bands of copper from ab initio pseudopotential calculations [40]. |

Figure Al.3.30. Theoretical frequency-dependent conductivity for GaAs and CdTe liquids from ab initio molecular dynamics simulations [42]. |

That said, the remarkable advances in computer hardware have made ab initio calculations feasible for small systems, provided that various technical details are carefiilly treated. A few examples of recent computations [c.199]

A few ab initio calculations are the main source of our current, very meagre knowledge of non-additive contributions to the short-range energy [91], It is unclear whether the short-range non-additivity is more or less important than the long-range, dispersion non-additivity in the rare-gas solids [28, 92], [c.200]

An example of a potential energy fiinction based on all these ideas is provided by the 10-parameter fiinction used [88] as a representation of ab initio potential energy curves for Fle-F and Ne-F [c.207]

Dykstra C E 1988 Ab initio Calculation of the Structures and Properties of Molecules (Amsterdam Elsevier) [c.210]

Tao F M and Klemperer W 1994 Accurate ab initio potential energy surfaces of Ar-HF, Ar-H20, and Ar-NHg J. Chem. Phys. 101 1129 5 [c.214]

Hu C H and Thakkar A J 1996 Potential energy surface for interactions between N2 and He ab initio calculations, analytic fits, and second virial coefficients J. Chem. Phys. 104 2541 [c.214]

Price S L and Stone A J 1980 Evaluation of anisotropic model intermolecular pair potentials using an ab initio SCF-CI surface Moi. Phys. 40 805 [c.217]

The relative acidities in the gas phase can be detennined from ab initio or molecular orbital calculations while differences in the free energies of hydration of the acids and the cations are obtained from FEP sunulations in which FIA and A are mutated into FIB and B A respectively. [c.516]

Many potential energy surfaces have been proposed for the F + FI2 reaction. It is one of the first reactions for which a surface was generated by a high-level ab initio calculation including electron correlation [47]. The [c.877]

At this point it is reasonable to ask whether comparing classical or quantum mechanical scattering calculations on model surfaces to asymptotic experimental observables such as the product energy and angular distributions is the best way to find the true potential energy surface for the F + FI2 (or any other) reaction. From an experimental perspective, it would be desirable to probe the transition-state region of the F + FI2 reaction in order to obtain a more direct characterization of the bending potential, since this appears to be the key feature of the surface. From a theoretical perspective, it would seem that, with the vastly increased computational power at one s disposal compared to 10 years ago, it should be possible to construct a chemically accurate potential energy surface based entirely on ab initio calculations, with no reliance upon empirical corrections. Quite recently, both developments have come to pass and have been applied to the F + FI2 reaction. [c.878]

Figure A3.7.6. Photoelectron spectrum of. Here the F is complexed to para-R - Solid curve experimental results. Dashed curve simulated spectrum from scattering calculation on ab initio surface. |

Figure Al.5.1 Potential energy curve for NeF based on ab initio calculations of Archibong et al |

Despite the recent successes of ab initio calculations, many of the most accurate potential energy surfaces for van der Waals interactions have been obtained by fitting to a combination of experimental and theoretical data. The fiitiire is likely to see many more potential energy surfaces obtained by starting with an ab initio surface, fitting it to a fimctional fonn and then allowing it to vary by small amounts so as to obtain a good fit to many experimental properties simultaneously see, for example, a recent study on morphing an ab initio potential energy surface for Ne-FIF [93]. [c.200]

There are many large molecules whose mteractions we have little hope of detemiining in detail. In these cases we turn to models based on simple mathematical representations of the interaction potential with empirically detemiined parameters. Even for smaller molecules where a detailed interaction potential has been obtained by an ab initio calculation or by a numerical inversion of experimental data, it is usefid to fit the calculated points to a functional fomi which then serves as a computationally inexpensive interpolation and extrapolation tool for use in fiirtlier work such as molecular simulation studies or predictive scattering computations. There are a very large number of such models in use, and only a small sample is considered here. The most frequently used simple spherical models are described in section Al.5.5.1 and some of the more common elaborate models are discussed in section A 1.5.5.2. section Al.5.5.3 and section Al.5.5.4. [c.204]

Parametrized representations of individual damping dispersion functions were first obtained [127] by fitting ab initio damping functions [74] for Ft-Fl interactions. The one-parameter dampmg fiinctions of Douketis et al are [127] [c.207]

Functional fonns based on the above ideas are used in the FIFD [127] and Tang-Toeimies models [129], where the repulsion tenn is obtained by fitting to Flartree-Fock calculations, and in the XC model [92] where the repulsion is modelled by an ab initio Coulomb tenn and a semi-empirical exchange-repulsion tenn Cunent versions of all these models employ an individually damped dispersion series for the attractive [c.207]

Bundgen P, Grein F and Thakkar A J 1995 Dipole and quadrupole moments of small molecules. An ab initio study using perturbatively corrected, multi-reference, configuration interaction wavefunctions J. Mol. Struct. (Theochem) 334 7 [c.210]

Thakkar A J, Hettema H and Wormer P E S 1992 Ab initio dispersion coefficients for interactions involving rare-gas atoms J. Chem. Phys. 97 3252 [c.212]

Woon D E 1994 Benchmark calculations with correlated molecular wavefunctions. 5. The determination of accurate ab initio intermolecular potentials for He2, Ne2, and A 2 J. Chem. Phys. 100 2838 [c.214]

Meuwly M and Hutson J M 1999 Morphing ab initio potentials a systematic study of Ne-HF J. Chem. Phys. 110 8338 [c.214]

Momany F A 1978 Determination of partial atomic charges from ab initio molecular electrostatic potentials. Application to formamide, methanol and formic acid J. Phys. Chem. 82 592 [c.216]

Alavi A 1996 Path integrals and ab initio molecular dynamics Monte Carlo and Molecular Dynamics of Condensed Matter Systems ed K Binder and G Ciccotti (Bologna SIF) [c.556]

Since taking simply ionic or van der Waals radii is too crude an approximation, one often rises basis-set-dependent ab initio atomic radii and constnicts the cavity from a set of intersecting spheres centred on the atoms [18, 19], An alternative approach, which is comparatively easy to implement, consists of rising an electrical eqnipotential surface to define the solnte-solvent interface shape [20], [c.838]

Miertus S, Scrocco E and Tomasi J 1981 Electrostatic interactions of a solute with a continuum. A direct utilization of ab initio molecular potentials for the provision of solvent effects Ohem. Rhys. 55 117-25 [c.864]

At the time the experiments were perfomied (1984), this discrepancy between theory and experiment was attributed to quantum mechanical resonances drat led to enhanced reaction probability in the FlF(u = 3) chaimel for high impact parameter collisions. Flowever, since 1984, several new potential energy surfaces using a combination of ab initio calculations and empirical corrections were developed in which the bend potential near the barrier was found to be very flat or even non-collinear [49, M], in contrast to the Muckennan V surface. In 1988, Sato [ ] showed that classical trajectory calculations on a surface with a bent transition-state geometry produced angular distributions in which the FIF(u = 3) product was peaked at 0 = 0°, while the FIF(u = 2) product was predominantly scattered into the backward hemisphere (0 > 90°), thereby qualitatively reproducing the most important features in figure A3.7.5. [c.878]

The idea that certain sites on a surface are especially active is connnon in the field of heterogeneous catalysis [61]. Often these sites are defects such as dislocations or steps. But surface site specificity for dissociation reactions also occurs on perfect surfaces, arising from slight differences in the molecule-surface bonding at different locations. This is so not only of msulator and semiconductor surfaces where there is strongly directional bonding, but also of metal surfaces where the electronic orbitals are delocalized. The site dependence of the reactivity manifests itself as a strong corrugation in the PES, which has been shown to exist by ab initio computation of the interaction PES for H2 dissociation on some simple and noble metal surfaces [, and Mj. [c.910]

Detailed analyses of the above experiments suggest that the apparent steps in k E) may not arise from quantized transition state energy levels [110.111]. Transition state models used to interpret the ketene and acetaldehyde dissociation experiments are not consistent with the results of high-level ab initio calculations [110.111]. The steps observed for NO2 dissociation may originate from the opening of electronically excited dissociation chaimels [107.108]. It is also of interest that RRKM-like steps in k E) are not found from detailed quantum dynamical calculations of unimolecular dissociation [91.101.102.112]. More studies are needed of unimolecular reactions near tln-eshold to detennine whether tiiere are actual quantized transition states and steps in k E) and, if not, what is the origin of the apparent steps in the above measurements of k E). [c.1035]

See pages that mention the term

**Ab initio**:

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Computational chemistry using the PC (2003) -- [ c.241 , c.277 , c.299 ]