  We report the complete set of elastic constants and the bulk modulus for
single crystal Pb(Mg1/3Nb2/3)O3 (PMN) at room temperature obtained from
Brillouin spectroscopy and molecular dynamics (MD) simulations. The bulk
modulus from Brillouin is found to be 103 GPa, in a good agreement with earlier
x-ray studies. We also derived the refractive index along all principal axes
and found PMN to be optically isotropic, with a refractive index of 2.52 +/-
0.02. PMN shows mechanical anisotropy with A=1.7. The MD simulations of PMN
using the random site model overestimate the elastic constants by 20-50 GPa and
the bulk modulus is 148 GPa, but the mechanical anisotropy matches the
Brillouin results of A = 1.7. We also determined the elastic constants for
various models of PMN and we find variation in the elastic constants based on
chemical ordering.
