  Quantum-mechanical ab initio calculations are performed to elucidate the
vibrational spectroscopic features of a common irradiation-induced defect in
diamond, i.e. the neutral vacancy. Raman spectra are computed analytically
through a Coupled-Perturbed-Hartree-Fock/Kohn-Sham approach as a function of
both different defect spin states and defect concentration. The experimental
Raman features of defective diamond located in the 400-1300 cm-1 spectral
range, i.e. below the first-order line of pristine diamond at 1332 cm-1 , are
well reproduced, thus corroborating the picture according to which, at low
damage densities, this spectral region is mostly affected by non-graphitic sp3
defects. No peaks above 1332 cm-1 are found, thus ruling out previous tentative
assignments of different spectral features (at 1450 and 1490 cm-1) to the
neutral vacancy. The perturbation introduced by the vacancy to the thermal
nuclear motion of carbon atoms in the defective lattice is discussed in terms
of atomic anisotropic displacement parameters (ADPs), computed from converged
lattice dynamics calculations.
