**5. Concluding remarks**

In this chapter, we have discussed the optimal configurations of the C60 and C70 molecules inside SWCNTs. First, the configuration of C60 and C70 fullerenes inside SWCNT is found to depend strongly on the nanotube diameter and does not depend significantly on tube chirality. In agreement with experiments, the C70 molecules adopt a lying orientation for small SWCNT diameters (below 1.356 nm), whereas a standing orientation is preferred for large diameters (above 1.463 nm). Between these diameters, an intermediate tilted configuration of C70s is found. For C60 peapods, C60s adopt a linear arrangement for SWCNT diameter lower than 1.45 nm and a zigzag configuration for diameters between 1.45 and 2.15 nm. In the diameter range between 2.15 and 2.23 nm, C60 molecules adopt a double helix arrangement in SWCNTs, whereas a layer of two molecules is preferred for diameters between 2.23 and 2.28 nm. Next, we have reported the calculated Raman spectra of peapods. The modes located in the BLM range are very sensitive to the encapsulation of fullerenes because major changes in frequencies and intensities are observed. The average intensity ratios between C60 and C70 Raman-active modes and the nanotube ones, as a function of the concentration molecules, have been analyzed, and a general good agreement is found between calculations and measurements. This good agreement supports the experimental method proposed to evaluate C<sup>60</sup> concentration inside SWCNTs.
