# Lennard Jones reference configuration¶

In this example, an instantaneous configuration is compared against published values for the potential energy.

First, the potential energy between two particles is compared to the analytical value. Next, the energy of an entire configuration is compared against the NIST SRSW reference calculations.

[1]:

import unittest
import feasst as fst

class TestMonteCarloLJRef(unittest.TestCase):
"""The FEASST implementation of the LJ potential is tested against known cases."""
def test_two_particle(self):
"""Test the LJ potential against analytical calculation of two particles"""
system = fst.lennard_jones()
displacement = 1.2345
system.get_configuration().update_positions(fst.Double2DVector([[0., 0., 0.], [displacement, 0., 0.]]))

# compute the energy of the system
system.energy()

# compute the expected analytical LJ and LRC energies
enlj = 4*(displacement**(-12) - displacement**(-6))
rcut = system.configuration().model_params().cutoff().value(0)
enlrc = (8./3.)*fst.PI*system.configuration().num_particles()**2/ \
system.configuration().domain().volume()*((1./3.)*rcut**(-9) - rcut**(-3))

# Compare the analytical results with the FEASST computed energies.
# The energies of the individual potentials (e.g., LJ and LRC) are stored as profiles with
# indices based on the order that the potentials were initialized.
# Thus, profile index 0 refers to LJ while 1 refers to LRC.
# In addition, the last computed value of the energy of all potentials is also stored.
self.assertAlmostEqual(enlj, system.stored_energy_profile()[0], 15)
self.assertAlmostEqual(enlrc, system.stored_energy_profile()[1], 15)
self.assertAlmostEqual(enlj + enlrc, system.stored_energy(), 15)

def test_srsw_ref_config(self):
"""Test the LJ potential against a configuration of 30 particles.
In particular, the 4th configuration of the LJ SRSW reference:
https://www.nist.gov/mml/csd/chemical-informatics-research-group/lennard-jones-fluid-reference-calculations
"""
system = fst.lennard_jones()
fst.install_dir() + "/plugin/configuration/test/data/lj_sample_config_periodic4.xyz",
system.get_configuration())
self.assertEqual(30, system.configuration().num_particles())
system.energy()
enlj = -16.790321304625856
enlrc = -0.5451660014945704
self.assertAlmostEqual(enlj, system.stored_energy_profile()[0], 15)
self.assertAlmostEqual(enlrc, system.stored_energy_profile()[1], 15)
self.assertAlmostEqual(enlj + enlrc, system.energy(), 15)


Run the tests.

[2]:

unittest.main(argv=[''], verbosity=2, exit=False)

test_srsw_ref_config (__main__.TestMonteCarloLJRef)
Test the LJ potential against a configuration of 30 particles. ... ok
test_two_particle (__main__.TestMonteCarloLJRef)
Test the LJ potential against analytical calculation of two particles ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.003s

OK

[2]:

<unittest.main.TestProgram at 0x7fd028ae1780>