Ab initio band structure calculations

In the following paragraphs we describe representative calculations of the nonlinear bulk susceptibilities which adopt the SOS scheme starting with early applications employing simplified models and moving toward semi-ab initio and ab initio calculations that have progressively appeared in parallel to the development of ab initio band structure calculations. A summary is given in Table V. 

ESCOM Science Publishers, Leiden, 1990, pp. 745-780. LCAO ab Initio Band Structure Calculations for Polymers. 

TABLE 4-6. Pseudo potential used for ab-initio band structure calculation. 

Self-consistent ab initio band-structure calculations using the augmented-spherical-wave method have been carried out by Coehoom (1990) for hypothetical YFen and YFei2-jM t (M=Ti, V, Cr, Mo, and W). The calculated value of magnetic moment per Fe atom is 2.02 Ub, in good agreement with experiment, particularly if one takes into account that the experimental total moment also contains a small orbital contribution. 

After a number of semiempirical crystal-orbital (CO) calculations (extended Hvickel and CNDO/2 CO calculations for further references see Table III of Andre ) Otto et performed the first ab initio band-structure calculations on different fluorinated polyethylenes. The calculations were conducted for all six different polyfluoroethylenes, namely (CFH-CHj), (CF2-CH2), (CFH-CFH),, (CFH-CFH), (CF2-CFH)jt, and (CF2), which can be obtained from polyethylene through fluorine substitution. 

The results of ab initio band-structure calculations of six different polyglycine helices have recently been reported by Otto et As expected, the widths of the conduction and valence bands of these helices depend strongly on their structural (geometrical) parameters, but the gap is in all cases still larger than those given in Table 2.8. For further details we refer to Tables I and II of Otto et al. 

The importance of electron correlation effects were taken into account in ab initio band structure calculations based on GW approximation which explicitly included many-body effects. In this approach, the electron self energy was computed via a truncated expansion in terms of a Green s function G and the screened Coulomb interaction W. 

Since the unit cells in homopolynucleotides and homopolypeptides are in most cases fairly large, calculations performed until now are mostly only minimal basis ab initio band structure calculations. There are a few cases when aperiodicity in DNA and proteins were treated and a pilot calculation has been performed to treat the effect of the surrounding water on a cytosine stack /3/. 

LARGE SCALE AB INITIO BAND STRUCTURE CALCULATIONS OF POLYNUCLEOTIDES AND POLYPEPTIDES... 

Assignment of such spectra and fitting of model potential parameters to the observed band frequencies yields the magnitudes of V3 and V6 in the two electronic states involved, either S, and S0, or D0 and S,. Franck-Condon modeling of relative band intensities then yields the relative phase of the potentials in the two electronic states. We typically fix the absolute phase of the potentials from ab initio electronic structure calculations on both the S0 and D0 states. This provides an overall consistency check as well. 

Wahnon P. and Tablero C. (2002), Ab-initio electronic structure calculations for metallic intermediate band formation in photovoltaic materials , Phys. Rev. B 65, 165115-16124. 

For each material reported in Table 1, and for each Hamiltonian, we have calculated the band structure in the equilibrium geometry for KNbOs have also repeated the calculation at a common structure with a=4.03 A for all the Hamiltonians. Following the description given earlier, from the ab initio band structure we have calculated the following TB parameters ... 

After a number of semiempirical (extended Huckel, CNDO/2,< > INDO,< and MINDO< >) calculations in 1970, Andre performed the first ab initio band-structure computation of an infinite polyethylene chain. Since this polymer is mass-produced in the plastics industry, the theoretical study of its properties (especially of its mechanical properties, see Chapter 10) is of great practical interest, which explains the large... 

To determine theoretically the shift of the Fermi level and the change in the density of states when (SN) is modified, the coherent potential approximation with a fc-dependent self-energy (see Section 4.3.1) was applied to calculate the density of states of the partially filled band of the mixed chain.< > For this calculation, the available ab initio band structures of the pure (SN) and pure (SNH) chains were first applied as input. 

NWChem (we tested Version 3.2.1) is a program for ab initio, band-structure, molecular mechanics, and molecular dynamics calculations. The DFT band-structure capability is still under development and was not included in the Linux version tested. NWChem is unique in that it was designed from scratch for efficient parallel execution. The user agreement is more restrictive than most, apparently because the code is still under active development. At the time of this book s publication, limited support was available for users outside of the EMSL facility. 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

The tetramethylammonium salt [Me4N][NSO] is obtained by cation exchange between M[NSO] (M = Rb, Cs) and tetramethylammonium chloride in liquid ammonia. An X-ray structural determination reveals approximately equal bond lengths of 1.43 and 1.44 A for the S-N and S-O bonds, respectively, and a bond angle characteristic bands in the IR spectrum at ca. 1270-1280, 985-1000 and 505-530 cm , corresponding to o(S-N), o(S-O) and (5(NSO), respectively. Ab initio molecular orbital calculations, including a correlation energy correction, indicate that the [NSO] anion is more stable than the isomer [SNO] by at least 9.1 kcal mol . ... 

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