PYTORCH (Machine Learning Collective Variables)

Overview

The PYTORCH module is an interface between PyTorch machine learning library and PLUMED. It implements the PYTORCH_MODEL class, which is a subclass of Function class. PYTORCH_MODEL provide the ability to load models defined in Pytorch and compiled with TorchScript.

For instance, this allows one to use the outputs of a neural network as collective variables, as done in [18] and in [19]. Furthermore, the PYTORCH_MODEL outputs can also be used as inputs for other collective variables and for data analysis tools.

Installation

This module is not installed by default. It requires the PyTorch C++ APIs (LibTorch) to be linked against PLUMED.

Warning
Note that Libtorch APIs are still in beta phase regarding stability, so there might be breaking changes in newer versions. Currently, versions between 1.8.* and 1.13.* are supported. Please note that if you want to link a different version it might be necessary to manually specify the required libraries within LIBS in configure. Furthermore, it is advised to use the same version of PyTorch and LibTorch to avoid compatibility issues.

Download LibTorch C++ API library

You can download the pre-built LibTorch library from their website. For example, the following script downloads the 1.13.1 version (CPU, with C++11 ABI compatibility).

wget https://download.pytorch.org/libtorch/cpu/libtorch-cxx11-abi-shared-with-deps-1.13.1%2Bcpu.zip 
unzip libtorch-cxx11-abi-shared-with-deps-1.13.1+cpu.zip ; 
LIBTORCH=${PWD}/libtorch

The location of the include and library files need to be exported in the environment. For convenience, we can save them in a file sourceme.sh inside the libtorch folder:

> echo "export CPATH=${LIBTORCH}/include/torch/csrc/api/include/:${LIBTORCH}/include/:${LIBTORCH}/include/torch:$CPATH" >> ${LIBTORCH}/sourceme.sh
> echo "export INCLUDE=${LIBTORCH}/include/torch/csrc/api/include/:${LIBTORCH}/include/:${LIBTORCH}/include/torch:$INCLUDE" >> ${LIBTORCH}/sourceme.sh
> echo "export LIBRARY_PATH=${LIBTORCH}/lib:$LIBRARY_PATH" >> ${LIBTORCH}/sourceme.sh
> echo "export LD_LIBRARY_PATH=${LIBTORCH}/lib:$LD_LIBRARY_PATH" >> ${LIBTORCH}/sourceme.sh
> . ${LIBTORCH}/sourceme.sh

Remember to source the sourceme.sh file in your ~/.bashrc or ~/.bash_profile file.

Configure PLUMED

In order to install the PYTORCH module when compiling PLUMED we need to (1) specify to look for libtorch (--enable-libtorch) and (2) enable the related module (--enable-modules=pytorch or also --enable-modules=all):

> ./configure --enable-libtorch --enable-modules=pytorch
Attention
To verify that the linking of LibTorch is succesful, one should look at the output of the configure command, which should report one of the following lines: checking libtorch without extra libs.. yes or checking libtorch with -ltorch_cpu -lc10... yes. If not, configure will display a warning (and not an error!) that says: configure: WARNING: cannot enable __PLUMED_HAS_LIBTORCH. In this case, it is recommended to examine the output of the above two commands in the config.log file to understand the reason (e.g. it cannot find the required libraries).

Additional notes

  • A compiler with C++14 support is required.
  • If you want to use the pre-cxx11 ABI LibTorch binaries the following flag should be added to the configure: CXXFLAGS="-D_GLIBCXX_USE_CXX11_ABI=0".

Usage

Currently, all features of the PYTORCH module are included in a single function: PYTORCH_MODEL

CVs with PyTorch: the mlcvs package

mlcvs is a Python package (under development) that can be used to optimize different kinds of neural-networks based CVs, e.g. that discriminate between states [18] or that approximate the slow dynamical modes of the system [19]. The CVs are optimized in Python and the resulting model is compiled with TorchScript, in order to allowed the models to be employed without Python dependencies.

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