# Running a simulation in OpenMM¶

In this example, we'e going to actually run a short simulation with OpenMM saving the results to disk with MDTraj's HDF5 reporter

Obviously, running this example calculation on your machine requires having OpenMM installed. OpenMM can be downloaded and installed from https://simtk.org/home/openmm.

Lets import some things we're going to need from mdtraj

In [1]:
import os
import mdtraj
import mdtraj.reporters


And a few things froms OpenMM

In [2]:
from simtk import unit
import simtk.openmm as mm
from simtk.openmm import app


First, lets find a PDB for alanine dipeptide, the system we'll be simulating. We happen to have one inlcuded in the mdtraj package for testing named "native.pdb". Under normal circumstances circumstances, you shouldn't have much need for mdtraj.testing.get_fn (unless you're contributing tests to mdtraj!)

In [3]:
import mdtraj.testing
topology = pdb.topology.to_openmm()


Lets use the amber99sb-ildn forcefield with implicit solvent and a langevin integrator. This is relatively "standard" OpenMM code for setting up a system.

In [4]:
forcefield = app.ForceField('amber99sbildn.xml', 'amber99_obc.xml')
system = forcefield.createSystem(topology, nonbondedMethod=app.CutoffNonPeriodic)
integrator = mm.LangevinIntegrator(330*unit.kelvin, 1.0/unit.picoseconds, 2.0*unit.femtoseconds)
simulation = app.Simulation(topology, system, integrator)


Set the initial positions to the "first frame" of the PDB file (it only has one frame). Note that pdb is an mdtraj trajectory pass in its positions to OpenMM just fine though.

In [5]:
simulation.context.setPositions(pdb.xyz[0])
simulation.context.setVelocitiesToTemperature(330*unit.kelvin)


Let's use one of the OpenMM reporters that mdtraj provides. This is the hdf5 reporter, which saves all kinds of information, including the topology, positions, energies, etc to disk. To visualize the h5 trajectory with a non-hdf5 enabled app like PyMol or VMD, you can use mdconvert on the command line to easily transform it to NetCDF, DCD, or any other format of your preference.

In [6]:
if not os.path.exists('ala2.h5'):
simulation.reporters.append(mdtraj.reporters.HDF5Reporter('ala2.h5', 1000))
simulation.step(100000)

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