Improved Description of Atomic Environments using Low-cost Polynomial Functions with Compact Support

Abstract

The prediction of chemical properties using Machine Learning (ML) techniques calls for a set of appropriate descriptors that accurately describe atomic and, on a larger scale, molecular environments. A mapping of conformational information on a space spanned by atom-centred symmetry functions (SF) has become a standard technique for energy and force predictions using high-dimensional neural network potentials (HDNNP). Established atom-centred SFs, however, are limited in their flexibility, since their functional form restricts the angular domain that can be sampled. Here, we introduce a class of atom-centred symmetry functions based on polynomials with compact support called polynomial symmetry functions (PSF), which enable a free choice of both, the angular and the radial domain covered. We demonstrate that the accuracy of PSFs is either on par or considerably better than that of conventional, atom-centred SFs, with reductions in force prediction errors over a test set approaching 50% for certain organic molecules. Contrary to established atom-centred SFs, computation of PSF does not involve any exponentials, and their intrinsic compact support supersedes use of separate cutoff functions. Most importantly, the number of floating point operations required to compute polynomial SFs introduced here is considerably lower than that of other SFs, enabling their efficient implementation without the need of highly optimised code structures or caching, with speedups with respect to other state-of-the-art SFs reaching a factor of 4.5 to 5. This low-effort performance benefit substantially simplifies their use in new programs and emerging platforms such as graphical processing units (GPU). Overall, polynomial SFs with compact support improve accuracy of both, energy and force predictions with HDNNPs while enabling significant speedups with respect to their well-established counterparts.