A sum over molecular orbitals scheme with an ab initio 6-31G basis se
t is used to calculate the static first hyperpolarisabilities, beta, o
f Li@C-60, an endohedral derivative of C-60, recently synthesised in s
izeable quantities. In the most stable form, that is when the Li atom
is similar to 1.5 Angstrom off the centre of the cage, the calculated
beta is comparable to that of the best organic materials and to that o
f push-pull fullerenes to-be-synthesised. Because of the orbiting char
acter 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 chi((2)) of this material will: (a) be betwe
en one and two orders of magnitude larger than the magnetically induce
d chi((2)) of pristine C-60; and (b) show a strong temperature depende
nce. Comparison with preliminary measurements shows satisfactory agree
ment. (C) 1998 Elsevier Science B.V. All rights reserved.