Tight-binding calculations

An extended Huckel calculation is a simple means for modeling the valence orbitals based on the orbital overlaps and experimental electron affinities and ionization potentials. In some of the physics literature, this is referred to as a tight binding calculation. Orbital overlaps can be obtained from a simplified single STO representation based on the atomic radius. The advantage of extended Huckel calculations over Huckel calculations is that they model all the valence orbitals. 

Changes in the bandgap values depending on these patterns are summarised in Table 1 [16], where it is shown that only armchair-type CNT can have zero bandgap at a certain bond-alternation pattern even if they have not isodistant bond patterns. It should be emphasised that actual bond pattern is decided only by the viewpoints of energetical stabilisation, which cannot be predicted by the Hiickel-type tight-binding calculation. 

Fig. 25 (A) Tight-binding calculation (dashed) and EHMO (plain) of the electronic conduction, for the naked anthracene (a) when the two NO2 groups are perpendicular to the molecule (b), when one N02 is rotated (c) and when the two N02 are rotated (d). (B) Tunneling current intensity for the NOR gate (a) and the AND gate (b) depending on the orientation angle of the two N02 groups...

Figure 3.2 Nearest-neighbor tight-binding calculation of the density of electronic states (DOS) as a function of energy for a graphene sheet (black), a metallic (9,0) SWNT (blue), and a semiconducting (10,0) SWNT (red).

The susceptibility data are normalized to the Pauli susceptibility of undimerized (CH). This correspond to a one-dimensional metal with a 10-eV-wide half-filled conduction band. Tight-binding calculations with no coulombic interactions give xP = 3.9 x 10 6 emu/mol. The nature of the conducting state in highly doped (CH)X (i.e., for y > 5 to 6%), as noted... 

Of course, in an actual tight-binding calculation, all such integrals are evaluated only at the high symmetry points in the BZ. Fitted parameters are then used to interpolate the band structure between these points. 

The intrasite Coulomb gap is completely unaccounted for in the Hartree-Fock theory (e.g. tight-binding calculations), since electron correlation is neglected in the... 

Fig. 3.3 shows one calculation of the valence band density of states of a-Si H compared with the equivalent results for crystalline Si (Biswas et al. 1990). This is a tight binding calculation using the 216 atom cluster model created by Wooten, Winer and Weaire (1985). The... 

Fig, 3.3. Tight binding calculations of the valence and conduction bands of crystalline and amorphous silicon. The result for amorphous silicon is obtained from a 216 atom cluster (Biswas et al. 1990). 

These are the only SdH results to date of a M(dmit)2 salt which show clearly that at least for these materials simple tight-binding calculations have to be done with great care. The determination of the real low-temperature crystal structure is essential. Further work for more M(dmit)2 compounds would be needed to get a better understanding of the electronic properties which could give some hints for the important factors governing the appearance of superconductivity in these systems. 

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