• Stiff neural ordinary differential equations

      Kim, Suyong; Ji, Weiqi; Deng, Sili; Ma, Yingbo; Rackauckas, Christopher (American Institute of Physics Inc., 2021-09-20)
      Neural Ordinary Differential Equations (ODEs) are a promising approach to learn dynamical models from time-series data in science and engineering applications. This work aims at learning neural ODEs for stiff systems, which are usually raised from chemical kinetic modeling in chemical and biological systems. We first show the challenges of learning neural ODEs in the classical stiff ODE systems of Robertson’s problem and propose techniques to mitigate the challenges associated with scale separations in stiff systems. We then present successful demonstrations in stiff systems of Robertson’s problem and an air pollution problem. The demonstrations show that the usage of deep networks with rectified activations, proper scaling of the network outputs as well as loss functions, and stabilized gradient calculations are the key techniques enabling the learning of stiff neural ODEs. The success of learning stiff neural ODEs opens up possibilities of using neural ODEs in applications with widely varying time-scales, such as chemical dynamics in energy conversion, environmental engineering, and life sciences. Neural Ordinary Differential Equations (ODEs) have been emerged as a powerful tool to describe a dynamical system using an artificial neural network. Despite its many advantages, there are many scientific and engineering examples where neural ODEs may fail due to stiffness. This study demonstrates how instability arises in neural ODEs during the training with the benchmark stiff system. Furthermore, we apply scaling to differential equations and loss functions to mitigate stiffness. The proposed technique in adjoint handling and equation scaling for stiff neural ODEs can be used in many biological, chemical, and environmental systems subject to stiffness. © 2021 Author(s).