Towards the parameterisation of the Hubbard model for salts of BEDT-TTF: A density functional study of isolated molecules

Abstract

We calculate the effective Coulomb repulsion between electrons/holes, U, and site energy for an isolated BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene] molecule in vacuo. U=4.2 \pm 0.1 eV for 44 experimental geometries taken from a broad range of conformations, polymorphs, anions, temperatures, and pressures (the quoted `error' is one standard deviation). Hence we conclude that U is essentially the same for all of the compounds studied. This shows that the strong (hydrostatic and chemical) pressure dependence observed in the phase diagrams of the BEDT-TTF salts is not due to U. Therefore, if the Hubbard model is sufficient to describe the phase diagram of the BEDT-TTF salts there must be significant pressure dependence on the intramolecular terms in the Hamiltonian and/or the reduction of the Hubbard U due to the interaction of the molecule with the polarisable crystal environment. The renormalised value of U is significantly smaller than the bare value of the Coulomb integral: F_0=5.2\pm0.1 eV across the same set of geometries, emphasising the importance of using the renormalised value of U. The site energy (for holes), xi=5.0\pm0.2 eV, varies only a little more than U across the same set of geometries. However, we argue that this plays a key role in understanding the role of disorder in ET salts in general and in explaining the difference between the beta_L and beta_H phases of beta-(BEDT-TTF)_2I_3 in particular.

13 pages, 6 figures, also see animations at http://www.youtube.com/watch?v=3K2kP8hWpZI, http://www.youtube.com/watch?v=wIz1cRsSdEs and http://www.youtube.com/watch?v=bNzUBAS6AFM, Expanded discussion of renormalisation effects. To appear in J. Chem. Phys

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• The origin of the difference in the superconducting critical temperatures of the beta_H and beta_L phases of (BEDT-TTF)_2I_3

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• Theories of phosphorescence in organo-transition metal complexes - from relativistic effects to simple models and design principles for organic light-emitting diodes
• Importance of Spin-Orbit Coupling in Organic BEDT-TTF and BEDT-TSF Salts
• Electronic properties of Fabre charge-transfer salts under various temperature and pressure conditions
• Models of organometallic complexes for optoelectronic applications
• Spin liquid phase in the semiclassical theory of the Heisenberg model on an anisotropic triangular lattice with ring exchange
• Multi-Orbital Molecular Compound (TTM-TTP)I_3: Effective Model and Fragment Decomposition
• Possible observation of the signature of the bad metal phase and its crossover to a Fermi liquid in K(BEDT-TTF)2Cu(NCS)2 bulk and nanoparticles by Raman scattering