ELSEVIER Synthetic Metals 85 (1997) 1631-1632 BEDT-TTF salts: microscopic parameters from ab-initio calculations A. FortunelliO, and A. Painellib a Istituto di Chimica Quantisica ed Energetica Molecolare de1 CNR Via de1 Risorgimento 35, I-561.26 Pisa, Italy b Dip. Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica; Uniuersitti di Parma Viale delle Scienze, I-43100, Parma, Italy Abstract Extensive ab initio calculations are performed on BEDT-TTF dimeric units singled out from the K-(BEDT- TTF)2Cu[N(CN)2]Br (ETBR) structure. From a careful analysis of the results obtained for dimers with total charge ranging from 0 to 4 we are able to get r&able estimates of the Hubbard model parameters. Keyurords: organic superconductors, ab initio quantum chemical methods and calculations The importance of Hubbard models (HM) in modeling CT organic crystals is hardly over-emphasized. However their applicability to this class of compounds has to be tested by comparison with experiment and/or with ab initio calculations. The theoretical evaluation of HM parameters represents an open problem. For BEDT-TTF crystals the hopping integrals (t) have been evaluated in the Extended Hiickel (EH) approach [l], whereas U and V parameters have been estimated based on AM1 semiempirical approach [2]. We have devised a simple strategy to derive all the parameters of HM from a set of ab initio calculations on a dimeric unit [3]. In the proposed approach we test the reliability of HM, and get a reliable and internally consistent set of parameters for HM. The analysis is carried on under the assumption of frozen site orbitals, i.e. we neglect the relaxation of the molecular orbit& that follows electron I I I I Fig.1 Schematic projection view of the BEDT-TTF layer. Molecular long axis lie perpedicularly to the drawing plane. Letters refer to the interactions discussed in the text. transfer. Relaxation is expected to produce only minor effects in systems where molecular units bear positive charges. In the ETBR layer we have singled out four dimeric Lmits, corresponding to the four principal interactions (bl, ba, p, and q) displayed in Fig.1. We consider the experimental geometry at 120 K [4], and use the GAUSSIAN-94 set of programs [5] with the &31G** basis set. For the bl, p, and q geometries we have performed RHF-SCF calculations with total charge Q = 0, 2, 4. After localization of the corresponding frontier orbitals [3] we have evaluated the ab initio matrix elements corresponding to the parameters: t, U = $(iJl + Uz), AU = $(Uz - U,), V, h = $(hll + hzz), Ah = i(h22 - hll) (hii is the on-site energy of the i-th site), X (the repulsion between site- and bond-charges), and W (the self-repulsion of bond-charges). The results are collected in Table I. Table 1 Ab initio parameters (eV) for localized frozen orbitals derived from RHF-SCF calculation at the bl, p, and q geometries, with total charge Q= 0,2,4. PQ h Ah 1 U V AU W x h 4 -13.97 0. -.272 5.44 2x3 0. 0.0035 -0.OU4G b, 2 -16.84 0. -279 6.40 3.26 P- 0.0051 0.0192 h 0 .1%2-l 0. 301 5.5s 3.14 0. 0.0052 O.Oli4 P 4 -13.40 -0.0516 -.I29 5.49 2.32 -0.0014 0.0011 0.0037 P 2 -15.76 -0.0302 -.I22 6.4s 2.46 0.0092 0.0010 0.0020 P 0 -16.90 O.CW? -.I35 5.57 2.45 -0.0057 0.0024 0.0030 P 4 -13.05 0.0463 7.034 5.53 1.96 -0.0034 0.0014 0.0035 Q 2 -15.2; 0.0%6 -.037 6.49 2.11 -0.OQO2 0.0006 0.0039 P 0 -16.33 0.0170 -.CMi 5.55 2.16 -0.0016 0.0036 0.0036 0379-6779/97/617.00 0 1997 Elsevier Science S.k Ail rights resewed PII SO379-6779(96)04526-2