"""
Naturalunit system.

The natural system comes from "setting c = 1, hbar = 1". From the computer
point of view it means that we use velocity and action instead of length and
time. Moreover instead of mass we use energy.
"""

from sympy.physics.units import DimensionSystem
from sympy.physics.units.definitions import c, eV, hbar
from sympy.physics.units.definitions.dimension_definitions import (
    action, energy, force, frequency, length, mass, momentum,
    power, time, velocity)
from sympy.physics.units.prefixes import PREFIXES, prefix_unit
from sympy.physics.units.unitsystem import UnitSystem


# dimension system
_natural_dim = DimensionSystem(
    base_dims=(action, energy, velocity),
    derived_dims=(length, mass, time, momentum, force, power, frequency)
)

units = prefix_unit(eV, PREFIXES)

# unit system
natural = UnitSystem(base_units=(hbar, eV, c), units=units, name="Natural system")