Ab initio electronic energy calculation

In crystals with the LI2 structure (the fcc-based ordered structure), there exist three independent elastic constants-in the contracted notation, Cn, C12 and 044. A set of three independent ab initio total-energy calculations (i.e. total energy as a function of strain) is required to determine these elastic constants. We have determined the bulk modulus, Cii, and C44 from distortion energies associated with uniform hydrostatic pressure, uniaxial strain and pure shear strain, respectively. The shear moduli for the 001 plane along the [100] direction and for the 110 plane along the [110] direction, are G ooi = G44 and G no = (Cu — G12), respectively. The shear anisotropy factor, A = provides a measure of the degree of anisotropy of the electronic charge... 

Ab initio electron correlated calculations of the equilibrium geometries, dipole moments, and static dipole polarizabilities were reported for oxadiazoles <1996JPC8752>. The various measures of delocalization in the five-membered heteroaromatic compounds were obtained from MO calculations at the HF/6-31G level and the application of natural bond orbital analysis and natural resonance theory. The hydrogen transfer and aromatic energies of these compounds were also calculated. These were compared to the relative ranking of aromaticity reported by J. P. Bean from a principal component analysis of other measures of aromaticity <1998JOC2497>. 

Conformational characteristics of PTFE chains are studied in detail, based upon ab initio electronic structure calculations on perfluorobutane, perfluoropentane, and perfluorohexane. The found conformational characteristics are fully represented by a six-state RIS model. This six-state model, with no adjustment of the geometric or energy parameters as determined from the ab initio calculations, predicts the unperturbed chain dimensions, and the fraction of gauche bonds as a function of temperature, in good agreement with available experimental values. 

In addition to experiments, a range of theoretical techniques are available to calculate thermochemical information and reaction rates for homogeneous gas-phase reactions. These techniques include ab initio electronic structure calculations and semi-empirical approximations, transition state theory, RRKM theory, quantum mechanical reactive scattering, and the classical trajectory approach. Although still computationally intensive, such techniques have proved themselves useful in calculating gas-phase reaction energies, pathways, and rates. Some of the same approaches have been applied to surface kinetics and thermochemistry but with necessarily much less rigor. 

O. Dolgounitcheva et al., Ab initio electron propagator calculations on the ionization energies of free base porphyrin, magnesium porphyrin and zinc porphyrin. J. Phys. Chem. 109, 11596-11601 (2005)... 

Density Functional Theory (DFT) has become a powerful tool for ab-initio electronic structure calculations of atoms, molecules and solids [1, 2, 3]. The success of DFT relies on the availability of accurate approximations for the exchange-correlation (xc) energy functional Exc or, equivalently, for the xc potential vxc. Though these quantities are not known exactly, a number of properties of the exact xc potential vxc(r) are well-known and may serve as valuable criteria for the investigation of approximate xc functionals. In this contribution, we want to focus on one particular property, namely the asymptotic behavior of the xc potential For finite systems, the exact xc potential vxc(r) is known to decrease like — 1/r as r —oo, reflecting also the proper cancellation of spurious self-interaction effects induced by the Hartree potential. 

In conclusion, we share the philosophy H. F. Schaefer III expressed in 1979 (which we believe is still valid in 1986, and very likely to be for the foreseeable future) We have been convinced for about five years that ab initio electronic structure calculations should not even attempt (except for the very simplest systems) to predict the entire potential energy surface . Since the success of a semiempirical method stems from the judicious combination of theory and experiment, we present a brief survey of the main theoretical methods in the remainder of this section. 

In I acl, the potential energy surfaces derived from ab initio electronic structure calculations demonstrated that the corrugation and anisotropy of the interaction potentials of molecules with surfaces, even with low-index metal surfaces, are much larger than previously assumed. Using these potential energy surfaces in realistic dynamical simulations confirmed the importance of taking the appropriate multidimensionality of the interaction dynamics into account [2, 3],... 

For more than three decades, van der Waals (vdW) complexes have become prototypes for studying energy transfer mechanisms and weak intermolecular forces. During these years, the understanding of vdW forces has expanded dramatically. With the development of experimental techniques such as supersonic nozzle expansion, and by performing more accurate ab initio electronic structure calculations, it became possible to study the structure and dynamics of vdW complexes in more detail. 

In this section we report our experimental findings relatively to three different reactions of CN radicals with simple alkynes, namely acetylene, methyl-acetylene and dimethyl-acetylene. We have selected these reactive systems for different reasons the reactions with C2H2 is the prototype for the class of reactions CN +- alkynes/polyynes, thus is expected to reveal key concepts for reactions with the higher members of the same series the reactions with methylacetylene and dimethylacetylene were selected to observe the effect of the H substitution with one or two alkyl groups. In all cases, the experimental results are discussed in the light of the ab initio electronic structure calculations for the stationary points of the relevant potential energy surfaces. 

Ab Initio Calculations of Structural and Elastic Properties. Equilibrium lattice constants, equilibrium volumes, as well as bulk and shear moduli can be assessed based on ab initio electron-structure calculations. They are obtained from the calculated total energies as a fimetion of volume in the bcc or fee crystal structure and from respective volume-conserving distortions of the lattice. In most cases, they agree well with experiments (Table 1.6). 

Ab initio Electron Propagator Calculations on Electron Detachment Energies of Fullerenes, Macrocyclic Molecules, and Nucleotide Fragments... 

Ab initio Electron Propagator Calculations Table 3.4 Vertical ionization energies of Q44, eV... 

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