# How I use OpenPathSampling¶

David W.H. Swenson

Usually these docs don’t have a byline, and they’re usually written in the plural “we.” That’s because most things in the docs are just facts, and are obviously agreed by all authors. However, this is about the specific workflow that I like to use, and others may have different opinions. That said, when I develop OPS, it is with my workflow in mind. So this workflow is particularly well-supported.

The essence of my workflow is based on the idea of splitting the simulation intro three stages: (1) setup; (2) sampling; and (3) analysis. In my workflow, setup and analysis are done in an interactive Python environment (e.g., Jupyter notebooks), while sampling is done in a simple script. There are a few ideas/guidelines that lead to my workflow:

• OPS has a powerful storage system that helps you track provenance of data. Whenever possible use it! Load objects from storage instead of creating new ones.

• Setting up a path sampling simulation can require care and attention. Set up simulations in an interactive environment (e.g., Jupyter notebook) and use the tools that OPS provides to perform sanity checks along the way.

• Analysis is best done interactively, using a tool like a Jupyter notebook that enables visualization of graphs and also stores the process in cells. This removes the overhead of redoing previous parts of the analysis every time a new idea to explore comes up. This also makes it easy to track the development of ideas, since the notebook acts as a sort of log.

The result is that my workflow tends to be something like this:

1. Setup: I perform the setup (defining engines, CVs, states/interfaces, networks, and move schemes) in a Jupyter notebook. Importantly, I provide these objects with names (which, for most objects, means using the .named() method at creation). I save all of these objects to a file, often called setup.nc.

2. Sampling: The sampling process may run for a long time, and often on a remote machine. So I create a regular (non-interactive) Python script to run this. Note that this script is very simple: it basically just needs to create the path simulator, so frequently it just involves loading things from the setup.nc and plugging them into the path simulator class. In some cases (e.g., changing GPU device index) you may need to modify the engine, or even create a new engine and therefore a new move scheme. But it might be better to anticipate this and put multiple objects (with different names) in the setup.nc.

3. Analysis: I do the analysis in Jupyter notebooks, for the reasons discussed above.

One feature that I often see users overlooking is the ability of OPS to re-use simulation objects. For example, if you want to do a committor analysis after performing TPS, you can load the load the states from a file instead of redefining them, and this guarantees that your state definitions are identical. The OPS storage subsystem has tools that enable us to ensure that objects are identical across multiple simulations, even with results stored in many different files. I choose to do this with a single common setup.py, although you could also load the states from the output file of the TPS simulation.

Our examples tend to partially reflect this approach. In particular, the alanine dipeptide TPS is an example where the actual sampling script is quite short and simple. In practice, I usually put equilibration with the sampling, while it is often with the setup in the examples. Also, the examples use a notebook for the sampling (to fit more easily with the other stages, and to mix in text descriptions).