# The Bouncy Particle Sampler

The Bouncy Particle Sampler (on arXiv) is an advanced MCMC algorithm making use of Gradient information. Initialized at a random position $x_0$ in the support of the target density $\pi$, and with a random direction $v_0$ (of euclidean length 1), the algorithm proceeds at iteration $i$ by moving to $x_{i+1} = x_i+\tau v_i$ where $\tau>0$ is a random length. The intuitive idea is

• the step size/learning rate $\tau$ should be high if the gradient points is similar to $v_{i}$ (similarity measured by their dot product), low otherwise
• after the move, the direction $v_{i+1}$ will be updated from $v_{i}$ using  the targets gradient at the new position $x_i$, by reflecting it on the hyperplane orthogonal to the gradient

At random times, the direction $v_{i+1}$ is refreshed to a uniformly drawn direction (without this, no irreducibility). While this algorithm is as mind-boggling as HMC at first reading (for a non-phycisist), it has the nice property that it only has one parameter tuning the frequency with which direction is refreshed, unlike Hamiltonian MC where step-size, number of steps and the Mass-Matrix for momentum have to be tuned. Also, there are no rejections of proposals as HMC (or any other MH based algorithm).

A variant of the algorithm partitions the sampling space when the target is a product of independent factors and there is a direction, position and step size for each factor. Thus, in this version an actual particle system (in the SMC sense) is set up, where the interaction between the particles is achieved by them colliding. Only particles belonging to overlapping factors collide. This part is very interesting but I think it could use a more intuitive exposition to the basic idea.
The paper does not manage to adapt the bouncy sampler to discrete spaces, but shows that one can just apply another MCMC step for the discrete part.

Also, the paper establishes that you can use Hamiltonian dynamics instead of straight line updates, but as far as I can tell the straight line version is used in the numerical experiments. I’m not sure why you would want Hamiltonian dynamics back, as this reintroduces the tuning problems of HMC (maybe less so when using NUTS).

In comparison to NUTS the paper reports that their algorithm performs better (of course). It is not entirely clear however if they used NUTS with adaptation of the mass matrix or without, which should make quite a difference. In their application example (in 10 Dimensions), they seem to not adapt the mass matrix of HMC, which is a pity. A comparison to Adaptive Metropolis with mass matrix and scale adaptation would have been nice, as this is often hard to beat regarding wall clock time. Not to mention its simplicity in implementation.

Overall a very interesting paper, though it doesn’t exactly give an easy exposition. One direction that might be interesting is wether refreshing the direction $v_{i+1}$ at random has to be done uniformly over directions. I suppose that it might be possible to do this non-uniformly, for example by drawing a vector from the empirical covariance of the target and normalizing it. But the correctness of this is likely harder to prove. Also, the advantage over HMC in terms of tuning parameters essentially vanishes in that case.

## 2 thoughts on “The Bouncy Particle Sampler”

1. Alexandre Bouchard-Côté says:

Dear Ingmar,

– The motivation for following Hamiltonian dynamics is that with respect with certain objectives, they can be done analytically (in particular, for the normal potential). So the idea is to perform the exact Hamiltonian dynamics for the ‘wrong’ normal potential, and correct the discrepancy via occasional bounces.

– While we do not use full matrix adaptation for the HMC baseline, we do use Stan’s default diagonal matrix adaptation (as well as NUTS, adaptation of gamma, delta, kappa, and all the other default adaptations used by Stan). Presumably the authors of Stan use this default since the dense matrix adaptation is computationally very expensive. I will give it a try, but I feel that in the 1000 dimensional examples this will be very slow. We provide in Section 5.4 a comparison of different HMC adaptation and metric configurations. Otherwise, we use Stan’s default options, just as we use the default parameter choice for our method (refreshment rate of 1).

– Regarding mix discrete-continuous space, the scheme we propose in Section 4.1 may look superficially like an MCMC algorithm, but it has the important difference that it preserves the rejection free property: the discrete state stays in one configuration for a random time, and then jumps to a distinct state. We will elaborate on that in the next version of the manuscript.

– One point which I apologize we didn’t make more explicit is that in contrast to what the ‘particle’ part of the name might suggest (and what the Peters and de With paper do), we do not use several particles (in the sense of replicates of the target space) in the local version. Instead, the variables of the local version are different dimensions of the target model.

Thanks again for the nice post!

Best,
Alex

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2. Ingmar says:

Ah, interesting. I must have been overloaded with physics at that point of reading. But if you follow Hamiltonian Dynamics of a Gaussian Approximation U_0, you could also sample from the Gaussian approximation directly and will get a pretty good sampler. Correction could be done either by using an IS framework instead of MCMC or you could Rao-Blackwellize Metropolis Hastings in the style of Robert & Casella or Douc & Robert if you want to stay in MCMC land.
About full-matrix adaptation: you’re right that this can be slow. Cholesky updates make sense here of course, but also you dont have to update at every sample. If you do it every 100 or 500 samples thats fine and gives strong performance improvements.
One thing I wonder is the following: does rejection-free mean better? Because for MH-style algorithms, higher acceptance rate doesn’t result in optimal diffusion necessarily.

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