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Fullerenes and their derivatives are a class of conjugated electron-accepting molecules that show promise for nonlinear optics in the realm of both the first and second hyperpolarizability.[1±4] At odds with other organic systems, they can exist in two doped states: the standard state in which the dopant is dispersed between the molecules and a more fascinating state in which the electron donor is present inside the carbon cage. In this combined experimental and theoretical study, it is shown that endohedral doping with Li increases the third-order response of C 60 by one order of magnitude. Endohedral doping of Buckminsterfullerene has the triple effect of transferring charge to the carbon cage of C 60 , triggering the distortion of the cage (both phenomena are present also in the exohedral doping) and generating a low potential energy surface for the motion of the Li atom.[5±9]As a consequence, any traces of Li@C 60 are due to averaging over such a motion and can be strongly temperaturedependent if the microscopic value of Li@C 60 is a function of the location of the alkali atom.[8±9] In this particular case, even a nearly constant value of g, the second hyperpolarizability concerning the position of the Li atom is of interest if one considers that exohedral doping was found to increase the second hyperpolarizability of C 60 by a factor of 65, [10] where complete charge transfer occurred. Since in the case of Li@C 60 , only 60 % of the electron goes from the dopant to the LUMO of C 60 , [4,5] a response intermediate between those of the neutral empty cage and the mono-anion may be expected. The availability of three points of g as a function of charging may therefore enable extrapolation of the response to larger charges. The recent breakthrough in the production of macroscopic quantities of Li@C 60[11±13] lead us to measure and calculate the second hyperpolarizability of Li@C 60 . The measurements were performed using z-scan [14,15] and degenerate-four-wave-mixing [16] (DFWM) procedures. The zscan technique provides both the sign and the magnitude of the real and imaginary parts of the third-order susceptibility while DFWM is used to determine its total magnitude. With the spectroscopic techniques, the macroscopic nonlinear optical response, i.e., the third-order susceptibility w, is measured. This quantity is the counterpart of the microscopic, molecular second-order hyperpolarizability. The computational model adopted was a non-standard procedure, which consists of a sum over molecular orbitals scheme. [17] In this approach, the orbitals are calculated ab initio and are then used to generate the transition dipole moments, which are multiplied together, weighed on the proper energy denominators, and finally added to give g. Despite the intense computational resources required, this model allowed us to systematically explore the second hyperpolarizability surface of Li@C 60 generated by the motion of the alkali atom 1.5 from the center of the cage, that is, the region of minimum energy conformation.To measure ...