First principles techniques have been employed to examine the isolated self
-interstitial in diamond. The assignment of the R2 EPR center to the self-i
nterstitial has been questioned because of the small fine structure term. W
e have calculated the spin-spin interaction tensor, which resolves the prob
lem, We have also modeled the optical proberties of I-1, and propose an inv
ersion-doubling process to give rise to the 6 meV splitting of the ground a
nd first excited states. The 1.685 and 1.859 eV lines can then be understoo
d as being transitions between states made up from the p-orbitals on the th
ree-fold coordinated C atoms. Our calculated piezospectroscopic tensor also
agress with experiment.