  We present the design of an optomechanical crystal nanobeam cavity that
combines finite-element simulation with numerical optimization, and considers
the optomechanical coupling arising from both moving dielectric boundaries and
the photo-elastic effect. Applying this methodology results in a nanobeam with
an experimentally realized intrinsic optical Q-factor of 1.2x10^6, a mechanical
frequency of 5.1GHz, a mechanical Q-factor of 6.8x10^5 (at T=10K), and a
zero-point-motion optomechanical coupling rate of g=1.1MHz.
