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For most constant-temperature OpenMM simulations, start with LangevinMiddleIntegrator when configurational properties such as structures or free energies are the focus. Add a barostat only if the target ensemble requires constant pressure. OpenMM’s Monte Carlo barostat changes the periodic box; it does not control temperature, so pair it with a thermostat or thermostated integrator set to the same temperature.
Choose the temperature-control method for your ensemble and observable
A thermostat or thermostated integrator is needed when you want to control temperature. The right choice depends on what you intend to measure, the simulation protocol, and the trade-offs you can accept; no one method is best for every OpenMM simulation. OpenMM’s running simulations guide recommends Langevin integration as a usual starting point for constant-temperature work.
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LangevinMiddleIntegrator for configurational sampling
LangevinMiddleIntegrator uses the LFMiddle discretization and is generally preferred over LangevinIntegrator when configurational sampling matters, including for structural properties and free energies. The OpenMM 8.0 integrator theory guide describes its trade-off: configurational properties tend to be sampled more accurately, while kinetic properties are less accurate. It also applies constraints twice per step, which can increase computational cost for constrained systems.
Other temperature-control choices
- VerletIntegrator: Use it alone for constant-energy dynamics. If you want to control temperature while using Verlet, OpenMM documents
AndersenThermostatas an alternative. - NoseHooverIntegrator: OpenMM’s guide describes its velocity-scaling approach as offering more accurate transport properties than the stochastic temperature control of LangevinMiddleIntegrator, at a slight efficiency cost.
Choose according to the ensemble and properties your protocol requires. In particular, distinguish configurational measurements from kinetic or transport properties rather than selecting an integrator by name alone.
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Decide whether the simulation needs a barostat
Use a barostat when the target ensemble requires constant pressure. A constant-volume simulation does not need one. OpenMM’s Monte Carlo barostat proposes changes to the size of the periodic box; its temperature parameter is used in calculating whether volume changes are accepted. It assumes temperature is controlled elsewhere and does not regulate it. The OpenMM User Guide therefore says to use it with a Langevin integrator or Andersen thermostat and to set the same temperature in both. Mismatched temperatures give incorrect results.
For example, the basic pairing is a thermostated integrator such as LangevinMiddleIntegrator plus MonteCarloBarostat. Specify the target temperature in each. If you are running constant-volume dynamics, omit the barostat instead of adding one by default.
Match the barostat to the periodic cell
Barostat choice determines which box dimensions can change. Select the degrees of freedom that match the physical model and simulation protocol.
- Uniform, isotropic scaling:
MonteCarloBarostatis the basic choice when scaling the whole periodic box uniformly is appropriate. - Independent axis changes:
MonteCarloAnisotropicBarostatscales axes independently and allows axis-specific pressures and fixed axes. - Membrane systems:
MonteCarloMembraneBarostatis designed for a membrane in the XY plane. It treats in-plane and normal directions differently and supports surface tension.
These options represent different physical models, not merely performance settings. Follow the protocol for your system when deciding which box motions are valid.
Set parameters from the protocol, not from example values
The OpenMM guide’s code examples use LangevinMiddleIntegrator(300*kelvin, 1/picosecond, 0.004*picoseconds) and MonteCarloBarostat(1*bar, 300*kelvin). These are illustrative values, not universal settings. Set temperature and pressure for the scientific question and protocol; choose friction and timestep with regard to the system and required accuracy. Specify units, and keep the barostat temperature equal to the integrator or thermostat temperature.
The MonteCarloBarostat API declaration gives a default volume-change proposal frequency of 25 time steps. The OpenMM Cookbook tutorial describes the barostatInterval argument and uniform box-vector scaling at that interval. This default describes implementation behavior, not a universally recommended physical setting. Check the API and documentation for your installed OpenMM version before relying on defaults; the cited integrator theory page is for OpenMM 8.0, while the running-simulations guide is the current guide.
Quick Recap
A practical setup checklist
- Define the ensemble. Use a constant-volume setup without a barostat, or include pressure coupling and a periodic box when constant pressure is required.
- Choose temperature control. For constant-temperature configurational sampling, consider
LangevinMiddleIntegratoras a general starting point, subject to your protocol and observable. Use another documented method when its trade-offs better match the target properties. - Choose box degrees of freedom. Select isotropic, anisotropic, or membrane pressure coupling to represent the system’s intended cell behavior.
- Set and verify parameters. Use protocol-specific temperature, pressure, friction, timestep, and barostat interval values. If using a barostat, confirm its temperature matches the temperature controller and verify version-specific defaults.
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