nitrogen.constants

Physical constants and reference data.

NITROGEN generally uses an Å/u/cm^-1 unit system, defined as follows

Dimension	Unit
[length]	\(\text{Å}\)
[mass]	\(\text{u}\)
[energy]	\(hc\times\text{cm}^{-1}\)
[temperature]	\(K\)
[electric charge]	\(e\)

All constants in the nitrogen.constants module are reported in these units, which are convenient for nuclear motion calculations. As of CODATA 2018, the units of length and energy have exact relationships to their SI counterparts. The unit of mass (u, unified atomic mass unit, i.e. one-twelth the mass of a carbon atom), however, does not have an exact SI value. The atomic unit of mass (the electron mass) has a similar status.

Physical constants
kb (kB)	Boltzmann constant \(k_B\)
h	Planck constant \(h\)
hbar	reduced Planck constant \(\hbar = h/(2\pi)\)
a0	Bohr radius \(a_0\)
c	speed of light
me	electron mass
NA	Avogadro constant \(N_A\)
memp	electron-to-proton mass ratio, \(m_e/m_p\)
mp	proton mass, \(m_p\)

Unit conversions
joule	joule, J
kJ	kilojoule per mole, kJ/mol
kcal	kilocalorie per mole, kcal/mol
eV	electron-volt, eV
Eh	hartree, \(E_h\)
hHz	energy equivalent of hertz, \(h \times \text{Hz}\)
second	second, s
t_au	atomic unit of time, \(\hbar/E_h\)
debye	debye, D

Look-up methods
constant_val()	Look up value by name.
constant_unc()	Look up uncertainty by name.
mass()	Look up atomic mass by element symbol.

Version information
constants_version()	Version string for constants data.
mass_version()	Version string for atomic mass data.

nitrogen.constants.charge(label)

Retrieve the nuclear charge (atomic number).

Parameters

label (str or list of str) – Atomic label(s).

Returns

The nuclear charge.

Return type

int or list of int

Notes

Literal values can be passed as label. They will be returned as is.

nitrogen.constants.constant_unc(name)

Retrieve a physical constant’s uncertainty. The uncertainty value does not reflect errors from finite precision numerical arithmetic.

Parameters

name (str) – The name of the constant

Returns

The uncertainty

Return type

float

nitrogen.constants.constant_val(name)

Retrieve a physical constant’s value

Parameters

name (str) – The name of the constant

Returns

The value

Return type

float

nitrogen.constants.constants_version()

Version information for physical constants data

nitrogen.constants.mass(label)

Retrieve atomic mass data.

Parameters

label (str or list of str) – Atomic label(s).

Returns

The atomic mass in u.

Return type

float or list of float

Notes

Literal float values can be passed as label. They will be returned as is.

nitrogen.constants.mass_version()

Version information for atomic mass data