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Constants
~~~~~~~~~
The following mathematical and physical constants are available through the ``h`` module:
::
h.PI 3.14159265358979323846
h.E 2.71828182845904523536
h.GAMMA 0.57721566490153286060 (Euler)
h.DEG 57.29577951308232087680 (deg/radian)
h.PHI 1.61803398874989484820 (golden ratio)
h.FARADAY 96484.56 (coulombs/mole) (legacy value)
h.FARADAY 96485.3321233100141 (modern value. derived from mole and electron charge)
h.R 8.31441 (molar gas constant, joules/mole/deg-K) (legacy value)
h.R 8.3144626181532395 (modern value. derived from boltzmann constant and mole)
h.Avogadro_constant 6.02214076e23 (codata2018 value, introduced version 8.0)
As of Version 8.0 (circa October, 2020) modern units are the default.
See :func:`nrnunit_use_legacy`
.. warning::
Constants are not treated specially by the interpreter and
may be changed with assignment statements. However a change of
``FARADAY``, ``R``, or ``Avogadro_constant`` generate a warning message
(default once).
Python warning messages can be managed with
``import warnings; warnings.filterwarnings(action)`` where useful actions
are ``"error"``, ``"ignore"``, ``"always"``, or ``"once"``.
If assignment takes
place due to execution of a hoc interpreter statement, the warning occurs
only once but cannot be avoided.
The legacy FARADAY is a bit different than the legacy faraday of the units database.
The legacy faraday in a :file:`.mod` mechanism is 96520.
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