thermodynamic_properties.pressure returning nan, other properties available in 3.6.0

[Ez-Dax]

Hello!

I was working on a simulation and noticed that this chunk of code:

simulation.state.thermalize_particle_momenta(filter=hoomd.filter.All(), kT=0.75)
thermodynamic_properties = hoomd.md.compute.ThermodynamicQuantities(filter=hoomd.filter.All())
simulation.operations.computes.append(thermodynamic_properties)
simulation.run(0)
thermodynamic_properties.pressure

returns nan. I went back to the tutorial notebooks, and in Introducing Molecular Dynamics, Initializing a Random System, if I change out one of the other properties for pressure in the cells demonstrating how to check them (degrees_of_freedom, kinetic_temperature, kinetic_energy) it also returns nan, which makes me think it isn't an issue in how I've set up my simulation.

According to the documentation, I believe that the pressure is computed based on a single frame rather than being differential, so that shouldn't be the issue. I would really like some guidance on why I'm not able to see what the pressure of my system is (and ideally how to fix it) if someone has an idea!

[joaander]

Set simulation.always_compute_pressure = True to compute the virials when they are not otherwise needed: https://hoomd-blue.readthedocs.io/en/v5.2.0/hoomd/simulation.html#hoomd.Simulation.always_compute_pressure

You also need particle interactions to provide a virial and an integrator to provide degrees of freedom, all of which are needed to compute the pressure and kinetic temperature.

Her is a minimal working example:

import hoomd

simulation = hoomd.util.make_example_simulation()
simulation.state.thermalize_particle_momenta(filter=hoomd.filter.All(), kT=0.75)
thermodynamic_properties = hoomd.md.compute.ThermodynamicQuantities(filter=hoomd.filter.All())
simulation.operations.computes.append(thermodynamic_properties)

integrator = hoomd.md.Integrator(dt=0.005)
cell = hoomd.md.nlist.Cell(buffer=0.4)
lj = hoomd.md.pair.LJ(nlist=cell)
lj.params[("A", "A")] = dict(epsilon=1, sigma=1)
lj.r_cut[("A", "A")] = 2.5
integrator.forces.append(lj)
nvt = hoomd.md.methods.ConstantVolume(
    filter=hoomd.filter.All(), thermostat=hoomd.md.methods.thermostats.Bussi(kT=0.75)
)
integrator.methods.append(nvt)
simulation.operations.integrator = integrator

simulation.always_compute_pressure = True

simulation.run(0)
print(thermodynamic_properties.pressure)
print(thermodynamic_properties.kinetic_temperature)