Normal-State Properties

Abstract

As discussed in Chapt. 2, weakly overlapping π-molecular orbitals provide a useful starting point for estimating the electronic states for molecular ma- terials. As an input for such tight-binding band-structure calculations, the molecular orbitals are usually calculated within the extended-Hückel approxi- mation which uses some empirical molecular parameters [169, 170, 82, 79, 81]. As has been discussed in more detail in [86], the extended-Hückel approxima- tion enables the calculation of the overlap integrals S and the corresponding transfer energies t between the π-orbitals of adjacent molecules. The latter quantity is a measure of the probability for an electron to hop from one molecule to the next. The so-derived extended-Hückel tight-binding bands have the usual cosine energy-dispersion relation

$$ E(k) = 2t_a \cos (k_a a) + 2t_b \cos (k_b b) + 2t_c \cos (k_c c), $$

(4.1)

where a, b, and c denote the intermolecular distances along the a-, b- and c- axes, k =(k _a , k _b , k _c ) the wave vector of the electron and t _a , t _b and t _c the directional-dependent transfer energies. For simplicity, a lattice with or- thorhombic symmetry has been assumed. This semi-empirical method has been very successfully applied in the case of the present molecular metals, see, e.g. [80, 82, 171, 96, 109, 172, 173, 174]