Optimizing neural network surrogate models: Application to black hole merger remnants

Surrogate models of numerical relativity simulations of merging black holes provide the most accurate tools for gravitational-wave data analysis. Neural network-based surrogates promise evaluation speedups, but their accuracy relies on (often obscure) tuning of settings such as the network architecture, hyperparameters, and the size of the training dataset. We propose a systematic optimization strategy that formalizes setting choices and motivates the amount of training data required. We apply this strategy on NRSur7dq4Remnant, an existing surrogate model for the properties of the remnant of generically precessing binary black hole mergers and construct a neural network version, which we label NRSur7dq4Remnant_NN. The systematic optimization strategy results in a new surrogate model with comparable accuracy and provides insights into the meaning and role of the various network settings and hyperparameters as well as the structure of the physical process. Moreover, NRSur7dq4Remnant_NN results in evaluation speedups of up to 8 times on a single CPU and a further improvement of 2000 times when evaluated in batches on a GPU. To determine the training-set size, we propose an iterative enrichment strategy that efficiently samples the parameter space using much smaller training sets than naive sampling. NRSur7dq4Remnant_NN requires $\mathcal{O}({10}^{4})$ training data, so neural-network-based surrogates are ideal for speeding up models that support such large training datasets, but at the moment cannot directly be applied to numerical relativity catalogs that are $\mathcal{O}({10}^{3})$ in size. The optimization strategy is available through the gwbonsai package.