How to define sampler_states in real system（in parallel tempering）

[xlimke]

I'm doing parallel tempering with openMM-plumed.

According to #547 the key method is:
def create(self, thermodynamic_state, sampler_states: list, storage, min_temperature=None, max_temperature=None, n_temperatures=None, temperatures=None, **kwargs):

sampler_states is referred to :

sampler_states : openmmtools.states.SamplerState or list. One or more sets of initial sampler states. If a list of SamplerStates, they will be assigned to replicas in a round-robin fashion.

In the example, sampler_states is defined as states.SamplerState(testsystem.positions).

However, I have trouble in defining sampler_states in real system instead of testsystem. I tried to define it as simulation.context.getState(). Then there is an error message:

File "/envs/openmm/lib/python3.9/site-packages/openmmtools/multistate/replicaexchange.py", line 244, in _pre_write_create

if len(sampler_states) > n_states:

TypeError: object of type 'State' has no len()

Is there any help in defining sampler_states in real system?

Thanks in advance,
Meng