Geodesic Lagrangian Monte Carlo over the space of positive definite   matrices: with application to Bayesian spectral density estimation
  We extend the application of Hamiltonian Monte Carlo to allow for sampling from probability distributions defined over symmetric or Hermitian positive definite matrices. To do so, we exploit the Riemannian structure induced by Cartan's century-old canonical metric. The geodesics that correspond to this metric are available in closed-form and---within the context of Lagrangian Monte Carlo---provide a principled way to travel around the space of positive definite matrices. Our method improves Bayesian inference on such matrices by allowing for a broad range of priors, so we are not limited to conjugate priors only. In the context of spectral density estimation, we use the (non-conjugate) complex reference prior as an example modeling option made available by the algorithm. Results based on simulated and real-world multivariate time series are presented in this context, and future directions are outlined. 
Andrew Holbrook, Shiwei Lan, Alexander Vandenberg-Rodes, Babak   Shahbaba
stat.CO