  We theoretically studied the electronic and electrical properties of metallic
and semiconducting peapods with encapsulated C_{60} (C_{60}@CNT) as a function
of the carbon nanotube (CNT) diameter. For exothermic peapods (CNT diameter >
11.8 A), only minor changes, ascribed to a small structural deformation of the
nanotube walls, were observed. These include a small electron charge transfer
(less than 0.10 electron) from the CNT to the C_{60} molecules and a poor
mixing of the C_{60} orbitals with those of the CNT. Decreasing the diameter of
the nanotube leads to a modest increase of the charge density located between
the C_{60}'s. More significant changes are obtained for endothermic peapods
(CNT diameter < 11.8 A). We observe a large electron charge transfer from
C_{60} to the tube, and a drastic change in electron transport characteristics
and electronic structure. These results are discussed in terms of pi-pi
interaction and C_{60} symmetry breaking.
