7 FIG. 4: (a) Real part of χ ( q , 0) for 14% doped LaOFeAs and (b) Δ χ$_{0}$ : the difference between the real part of χ for the doped and undoped LaOFeAs. The results are plotted on the basal plane. features (structures around Z and R points in otherwise flat response function) are also visible in the top plane (Fig. 3(b)). These main features of the response function can be explained on the basis of the electronic structure of LaOFeAs and by comparison with the non-interacting electron gas $^{29}$. The flat region near the zone center (Γ) is due to self-nesting of the FS cylinders. This scenario is similar to the 2D electron gas where the circular FS leads to a flat response with a sharp drop to zero at 2 k$_{F}$ . In the present case, however, this drop is smooth, due to different sizes and irregularities in shape of the FS cylinders. The broad peak in the χ at M -point is due to the strong nesting between hole and electron like FS sheets. The presence of a perfect nesting, which is not realised in this material $^{6,11}$, would give a diverging response at this point. Doping (see Fig. 4(a)) leads to asymmetric change in the size of the electron and the hole FS cylinders which manifests itself as a crater-like structure around Γ-point which continuously increases on going towards M / X points in the BZ. This monotonous increase of χ$_{0}$ with | q | indicates the predominance of inter-band transitions. As this inter-band nesting becomes less perfect upon doping (worse matching of e and h FS), it produces a volcano-like feature around the M -point. This feature, of the maximum being shifted away from the M -point, is indicative of a possibility of an incommensurate spin configuration in this material. Interestingly, this incommensuration has been recently predicted from two different ab-initio calculations $^{6,11}$. The flatness of χ$_{0}$ ( q , 0) in the remaining q − space is consistent with the fact that all the competing magnetic structures lie within a small energy range, with in plane q -vector. In order to understand the evolution of the features around the Γ and M points as a function of doping it is instructive to consider the contribution from different energy- wavevector regions to χ$_{0}$ . To study this we separate the contributions of different intra- and