Simulator Functions

As described in the API, the sim_f is called by a libEnsemble worker via a similar interface to the gen_f:

out = sim_f(H[sim_specs["in"]][sim_ids_from_allocf], persis_info, sim_specs, libE_info)

In practice, most sim_f function definitions written by users resemble:

def my_simulator(H, persis_info, sim_specs, libE_info):

Where sim_specs is a dictionary containing pre-defined parameters for the sim_f, and the other parameters serve similar purposes to those in the gen_f. Valid simulator functions can accept a subset of the above parameters.

The pattern of setting up a local H, parsing out parameters from sim_specs, performing calculations, and returning the local H with persis_info should be familiar:

batch_size = sim_specs["user"]["batch_size"]
local_H_out = np.zeros(batch_size, dtype=sim_specs["out"])

... # Perform simulation calculations

return local_H_out, persis_info

Between the output array definition and the function returning, any level and complexity of computation can be performed. Users are encouraged to use the executor to submit applications to parallel resources if necessary, or plug in components from other libraries to serve their needs.

Simulator functions can also return a calc_status integer attribute from the libensemble.message_numbers module to be logged.

Descriptions of included simulator functions can be found here.

The Simple Sine tutorial is an excellent introduction for writing simple user functions and using them with libEnsemble.


libEnsemble’s Executors are commonly used within simulator functions to launch and monitor applications. An excellent overview is already available here.

See the Executor with Electrostatic Forces tutorial for an additional example to try out.

Persistent Simulators

Although comparatively uncommon, simulator functions can also be instantiated and written in a persistent fashion. See the here for a general API overview of writing persistent generators, since the interface is largely identical. The only differences are to include EVAL_SIM_TAG when instantiating a PersistentSupport class instance, and to return FINISHED_PERSISTENT_SIM_TAG when the simulator function returns.


An example routine using a persistent simulator can be found in test_persistent_sim_uniform_sampling.