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3.7 effective model for the transport properties 45β [94]. k αβ , for a fixed temperature, is proportional to J 2 αβ ,where J αβ is the transfer integral between the molecularelectronic orbitals [95]. J αβ depends on the relative positionand orientation of the two molecules. J αβ can be calculatedfrom DFT for the case of two infinite thiophene chains orientedalong x direction and stacked along z.In Figure 3.15 is reported in green the J αβ dependenceon the relative y shift of the two chains with respect tothe dependence of the transfer integral J 0 calculated at theequilibrium distance d 0 .Figure 3.15.: Comparison between the relative transfer integral J αβ /J 0as computed approximating thiophene rings by ellipses(red line) and first-principles calculations (green line).The maximum J αβ is found at zero shift (i.e. maximumoverlap area) and by increasing y up to y = 3.6 Å itdecreases monotonically to zero. In the same figure is reportedin red the stacking parameter Θ ⊥ calculated by usingellipses with eccentricity ɛ = 1.15, chosen so as to bestfit the first-principle calculations. Small differences (fewpercents) are found only at shifts 4-6 Å, but the overallagreement is good.As for the J αβ dependence on the π − π distance d betweenthe two molecules J αβ /J 0 = exp(−γ(d − d 0 )/d 0 ) [94]has been used, where γ is a fitting parameter. In conclusion,the geometrical stacking parameter Θ ⊥ can be used