Implementation of neural network potentials for coarse-grained models

Dr. Andreas Singraber

Host beneficiary: TU Wien and University of Vienna, Austria

 

Description

Neural network potentials (NNPs) [1] have demonstrated the effectiveness of
 machine-learning tools in the context of atomistic simulations. This approach,
which is based on artificial neural networks trained to accurately reproduce ab 
initio potential energy surfaces, offers two major advantages. First, with
 respect to the underlying reference method the computational effort to calculate energies and forces is drastically reduced, which allows to sample large system
 sizes and long time scales in molecular dynamics simulations. In addition, 
unlike empirical potentials, the neural network at the very heart of the method 
is not limited by an approximate functional form but can flexibly adjust to the 
reference potential energy surface. Recently, it has been proposed to extend 
the machine-learning potential approach to the construction of coarse-grained 
models [2]. In this pilot project we apply this methodology to neural network 
potentials and develop a software based on the existing package n2p2
. We intend to construct a coarse-grained model for dendrimer-like DNA 
molecules [3] in order to demonstrate the implementation.

[1] Behler, J.; Parrinello, M. Generalized Neural-Network Representation of
 High-Dimensional Potential-Energy Surfaces. Phys. Rev. Lett. 2007, 98 (14),
146401.

[2] Zhang, L.; Han, J.; Wang, H.; Car, R.; E, W. DeePCG: Constructing 
Coarse-Grained Models via Deep Neural Networks. J. Chem. Phys. 2018, 149 (3),
034101.

[3] Jochum, C.; Adžić, N.; Stiakakis, E.; Derrien, T. L.; Luo, D.; Kahl, G.;
Likos, C. N. Structure and Stimuli-Responsiveness of All-DNA Dendrimers: Theory
 and Experiment. Nanoscale 2019, 11 (4), 1604–1617.

Development Plan

List of Tasks

  • 1. Implement Python tools to generate coarse-grained from fully atomistic data sets.
  • 2. Devise and implement a procedure to estimate the effectiveness of the coarse-grained description with atomic environment descriptors.
  • 3. Train and evaluate NNP-CG models for simple (water) and complex (DNA dendrimer) systems.
  • 4. Improve CG models by inclusion of additional degrees of freedom (e.g. orientation, more particle types).
  • 5. Allow large-scale MD simulations on GPUs via LAMMPS and Kokkos.

List of Modules

Descriptor analysis

Status: Work in Progress

Expected delivery date: September 2020

Description: The overall goal of the analysis is to show qualitatively whether there is a correlation between the raw atomic environment descriptors (and their
derivatives) and the atomic forces. If no or very little correlation can be found we can assume that the descriptors do not encode enough information to
construct a (free) energy landscape. On the other hand, if "similar" descriptors correspond to "similar" forces there is a good chance that a machine learning
algorithm is capable of detecting this link and a machine learning potential can be fitted. In order to find a possible correlation between descriptors and
forces the following approach is used: First, a clustering algorithm (k-means or HDBSCAN) searches for groups in the high-dimensional descriptor space of all
atoms. Then, for every detected cluster the statistical distribution of the corresponding atomic forces is compared to the statistics of all remaining
atomic forces. A hypothesis test (Welch's t-test) is applied to decide whether the link between descriptors and forces is statistically significant. The
percentage of clusters which show a clear link is then an indicator for a good descriptor-force correlation.

Symmetry Function Memory Footprint Reduction

Status: Ready (to be merged into E-CAM documentation)

Expected delivery date: July 2020

Description: This module improves memory management in n2p2
. More specifically, a new strategy to store symmetry function derivatives is implemented. In this way the memory footprint during training is drastically reduced. The idea is to exploit that in a multi-element system for specific combinations of neighboring atoms the symmetry function derivatives always equal zero. Hence, by taking these element combination relations automatically into account a significant portion of the memory usage can be avoided. Depending on the symmetry function setup savings of about 30 to 50% can be achieved for typical systems.

Additional degrees of freedom for NNPs

Status: Upcoming (if necessary)

Expected delivery date: -

Description: Integration of additional degrees of freedom such as orientation vectors requires substantial changes in the n2p2 code, in particular for the training procedure.

GPU support for NNP-CG models via LAMMPS/Kokkos

Status: Upcoming

Expected delivery date: End of 2020

Description: A working LAMMPS interface which will take advantage of GPUs via the Kokkos library will allow for highly-efficient parallel simulations with NNP-CG models.

Tools collection for creating NNP-CG models

Status: Upcoming

Expected delivery date: End of 2020

Description: This module collects code improvements and actual tools extending n2p2
 which simplify the process of generating coarse-grained data sets from fully atomistic configurations.

Published Results

Outreach Material