On-site coulomb energies

The intraatomic interactions, on the other hand, are stronger the weaker the covalent mixing, which makes the on-site coulomb energies... 

It should be added that the expansion of the tr-conjugating system and/or the introduction of Se atoms with greater polarizability (as shown in BETS or TTPs) reduce the effective on-site Coulomb energy U. For example, the V values for the TTP-type donors estimated from electrochemical and optical measurements are considerably small. (TTM-TTP)l3 is a 1 1 salt with a highly one-dimensional half-filled band. Although 1 1 salts were believed to be non-metal due to the Coulomb interaction (Mott-insulator), this salt shows metallic behavior down to about 160 K. This system is regarded as a small-f/ case of a one-dimensional conductor with an inherently half-filled band [27]. 

The values of p(0) and p(l) and their ratio are in comparable magnitude with those predicted theoretically for a moderate on-site Coulomb energy of several eV [1,112-115]. Moreover, they are quantitatively reproduced by the calculation incorporating the... 

When the effective on-site Coulomb repulsive energy (Geff) of the solid composed of Tt-radical molecules is smaller than the bandwidth (W), then the solid becomes a half-filled metal provided that the molecules stack uniformly without dimerization and can be described by a band picture. So far, no such radical molecules have been prepared. In order to decrease Ues and stabilize radical molecules chemically, a push-pull effect and an extension of the re-system have been implemented, though a large U ff and high reactivity (polymerization) are stiU crucial for the metallic transport. Table 2 summarizes selected organic conductors of neutral 7t-radical molecules. 

Metallic properties are also observed in columnar stacked complexes in which the intrachain separation is too long for conduction to be associated with the formation of a band from overlap of the metal dz2/pz orbitals. For these complexes the conduction process involves carriers present in the delocalized MOs extending over die whole complex. The conduction process in these compounds may be described by a hopping mechanism in which the on-site Coulomb repulsion energy has been reduced by the delocalization of the charge over the whole molecule. 

Likewise the Hubbard model the periodic Anderson model (PAM) is a basic model in the theory of strongly correlated electron systems. It is destined for the description of the transition metals, lanthanides, actinides and their compositions including the heavy-fermion compounds. The model consists of two groups of electrons itinerant and localized ones (s and d electrons), the hybridization between them is admitted. The model is described by the following parameters the width of the s-electron band W, the energy of the atomic level e, the on-site Coulomb repulsion U of d-electrons with opposite spins, the parameter V of the... 

When the long-range Coulomb interaction is small, one can take account of only the on-site Coulomb interaction energy U between two electrons on a molecule. This electronic system is described in terms of the Hubbard model. Theoretical studies have shown that for UtW > 1 the electrons undergo the Mott transition which does not necessarily involve any structural changes. The electrons are localized with equal distance. They are apparently the same as the Wigner crystal described above. It is shown that the Mott transition is easy to occur when the charge density is l/molecule or I/site. 

In the PPP case we have to deal with a three-dimensional parameter space, namely the resonance integral B0, the electron-phonon coupling constant a, and the on-site Coulomb repulsion integral y0. Thus we varied the values of B and a and calculated the energy of butadiene in the model at u,/u0 = 0 (p=l), 0.5 (p=2), 1.0 (p=3) and 1.5 (p=4). Note, that p is not a site index here. The values obtained (Eppp(up)) are compared with the corresponding Ecc Uj) energies. The energies are computed relative to Up=0. Then we calculated... 

In this context the HF Hamiltonian is of particular interest. HF theory is distinguished from the LDA in that it attempts to compute the ground state wave function, whereas LDA computes the ground state density and energy. The HF approximation therefore contains explicit interactions between the orbitals and is automatically self-interaction corrected. The on-site Coulomb interaction is therefore treated quite naturally and a qualitatively correct description of the ground states is obtained. The density of states of NiO projected onto the Ni Eg, J2g and O orbitals computed in the HF approximation is shown in Fig. 8.3. 

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