# Engines¶

## The Trajectory object¶

Before we talk about adaptivity, let’s have a look at possibilities to generate trajectories.

We assume that you successfully ran a first trajectory using a worker. Next, we talk about lot’s of ways to generate new trajectories.

You will do this in the beginning. Remember we already have a PDB stored from setting up the engine. if you want to start from this configuration do as before

1. create the Trajectory object you want
3. submit the task to craft the object into existance on the HPC

A trajectory contains all necessary information to make itself. It has

1. a (hopefully unique) location: This will we the folder where all the files that belong to the trajectory go.
2. an initial frame: the initial configuration to be used to tell the MD simulation package where to start
3. a length in frames to run
4. the Engine: the actual engine I want to use to create the trajectory.

Note, the Engine is technically not required unless you want to use .run() but it makes sense, because the engine contains information about the topology and, more importantly information about which output files are generated. This is the essential information you will need for analysis, e.g. what is the filename of the trajectory file that contains the protein structure and what is its stride?

Let’s first build a Trajectory from scratch

file_name = next(project.traj_name)              # get a unique new filename

trajectory = Trajectory(
location=file_name,                          # this creates a new filename
frame=pdb_file,                              # initial frame is the PDB
length=100,                                  # length is 100 frames
engine=engine                                # the engine to be used
)


Since this is tedious to write there is a shortcut

trajectory = project.new_trajectory(
frame=pdb_file,
length=100,
engine=engine,
number=1          # if more then one you get a list of trajectories
)


Like in the first example, now that we have the parameters of the Trajectory we can create the task to do that.

### OpenMMEngine¶

Let’s do an example for an OpenMM engine. This is simply a small python script that makes OpenMM look like a executable. It run a simulation by providing an initial frame, OpenMM specific system.xml and integrator.xml files and some additional parameters like the platform name, how often to store simulation frames, etc.

engine = OpenMMEngine(
pdb_file=pdb_file,
args='-r --report-interval 1 -p CPU'
).named('openmm')


We have now an OpenMMEngine which uses the previously made pdb File object and uses the location defined in there. The same for the OpenMM XML files and some args to run using the CPU kernel, etc.

Last we name the engine openmm to find it later.

engine.name


Next, we need to set the output types we want the engine to generate. We chose a stride of 10 for the master trajectory without selection and a second trajectory with only protein atoms and native stride.

Note that the stride and all frame number ALWAYS refer to the native steps used in the engine. In out example the engine uses 2fs time steps. So master stores every 20fs and protein every 2fs

engine.add_output_type('master', 'master.dcd', stride=10)

 Engine() An generator for trajectory simulation tasks Trajectory(location, frame, length[, engine]) Represents a trajectory File on the cluster OpenMMEngine(system_file, integrator_file, ...) OpenMM Engine to be used by Adaptive MD