The hyperpolarisability of an endohedral fullerene: Li@C₆₀

A sum over molecular orbitals scheme with an ab initio 6-31G* basis set is used to calculate the static first hyperpolarisabilities, β, of Li@C₆₀, an endohedral derivative of C₆₀, recently synthesised in sizeable quantities. In the most stable form, that is when the Li atom is ~1.5 Å off the centre of the cage, the calculated β is comparable to that of the best organic materials and to that of push-pull fullerenes to-be-synthesised. Because of the orbiting character of the motion of Li inside the carbon cage, and the substantial variation of the hyperpolarisability with the location of the Li atom, it is suggested that the χ⁽²⁾ of this material will: (a) be between one and two orders of magnitude larger than the magnetically induced χ⁽²⁾ of pristine C₆₀; and (b) show a strong temperature dependence. Comparison with preliminary measurements shows satisfactory agreement.