## Abstract

We present ab initio two-dimensional extended Hubbard-type multiband models for EtMe _{3}Sb[Pd(dmit) _{2}] _{2} (where dmit is 1,3-dithiole-2-thione-4,5-dithiolate) and κ-(BEDT-TTF) _{2}Cu(NCS) _{2} [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene] after a downfolding scheme based on the constrained random-phase approximation (cRPA) and maximally localized Wannier orbitals, together with the dimensional downfolding. In the Pd(dmit) _{2} salt, the antibonding state of the highest occupied molecular orbital (HOMO) and the bonding/antibonding states of the lowest unoccupied molecular orbital (LUMO) are considered to be the orbital degrees of freedom, while, in the κ-BEDT-TTF salt, the HOMO-antibonding/bonding states are considered. Accordingly, a three-band model for the Pd(dmit) _{2} salt and a two-band model for the κ-(BEDT-TTF) salt are derived. We derive single-band models for the HOMO-antibonding state for both of the compounds as well. The HOMO antibonding band of the Pd(dmit) _{2} salt has a triangular structure of the transfers with a one-dimensional anisotropy, in contrast to the nearly equilateral triangular structure predicted in the extended Hückel results. The ratio of the larger interchain transfer t _{b} to the intrachain transfer t _{a} is around t _{b}/t _{a}∼0.82. Our calculated screened onsite interaction U and the largest offsite interaction V are ∼0.7 and ∼0.23 eV, respectively, for EtMe _{3}Sb[Pd(dmit) _{2}] _{2} and ∼0.8 and ∼0.2 eV for κ-(BEDT-TTF) _{2}Cu(NCS) _{2}. These values are large enough compared to transfers t as ∼55 meV for the Pd(dmit) _{2} salt and ∼65 meV for the κ-BEDT-TTF one, and the resulting large correlation strength (U-V)/t∼10 indicates that the present compounds are classified as the strongly correlated electron systems. In addition, the validity whether the present multiband model can be reduced to the single-band model for the HOMO-antibonding state, widely accepted in the literature, is discussed. For this purpose, we estimated the order of vertex corrections ignored in the cRPA downfolding to the single-band model, which is given by W′/D, where W′ is a full-screened-interaction matrix element between the HOMO-antibonding and other bands away from the Fermi level (namely, HOMO-bonding or LUMO-bonding/antibonding bands), whereas D is the energy distance between the Fermi level and the bands away from the Fermi level. In the present materials, W′/D estimated as 0.3-0.5 signals a substantial correction and thus the exchange process between the low-energy HOMO-antibonding and other bands away from the Fermi level may play a key role to the low-energy ground state. This supports that the minimal models to describe the low-energy phenomena of the organic compounds are the multiband models and may not be reduced to the single-band model.

Original language | English |
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Article number | 205117 |

Journal | Physical Review B - Condensed Matter and Materials Physics |

Volume | 86 |

Issue number | 20 |

DOIs | |

Publication status | Published - 2012 Nov 16 |

Externally published | Yes |

## ASJC Scopus subject areas

- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics

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_{2}and κ-(BEDT-TTF)

_{2}Cu(NCS)

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