# Constants

Specifies which set of physical constants to use. Note that using an older set should only be done in order to compare results with earlier versions of Gaussian.

1998
Constants from [428] and references therein. This is the default

1986
Constants used in Gaussian 88 through Gaussian 98, from [429,430].

1979
Constants used in Gaussian 80 through Gaussian 86, mostly from [431]. The OldConstants keyword is a synonym for Constants=1979.

### CURRENT VALUES

Here are summarized various conversion factors and physical constants used by Gaussian 03 in converting from standard to atomic units. All quantities used in calculations inside Gaussian are in atomic units; the conversion factors are used only when processing input or generating printed output.

Raw Constants. The constants which are stored directly in the program are:

1 Bohr (a0) = 0.5291772083 Å [428]

1 Atomic mass unit (amu) = 1.66053873 x 10-27 kilograms [428]

1 Electron charge (e) = 4.803242 x 10-10 ESU [432] = 1.602176462 x 10-19 Coulombs [428]

Planck's constant (h) = 6.62606876 x 10-34 Joule-secs [428]

Avogadro's number (NA) = 6.02214199 x 1023 [428]

1 Calorie = 4.184 Joules [431]

1 Hartree = 4.3597482 x 10-18 Joules [429]

Speed of light (c) = 2.99792458 x 1010 cm-sec-1 [431]

Boltzman constant (k) = 1.380603 x 10-23 Joules-degree-1 [428]

Inverse fine structure constant (α-1) = 137.03599976 [428]

Molar volume of ideal gas at 273.15 K = 0.022413996 m3 [428]

Proton rest mass = 1.67262158 x 10-27 kg [428]

Electron magnetic moment = 9.28476362 x 10-24 J-T-1 [428]

Free electron g-factor = 2.002319304386 (dimensionless) [428]

Conversion Factors. The following useful conversion factors can be derived from the above:

Electron mass = 0.910938 x 10-30 kg

Proton mass = 1836.1527 electron mass

1 Atomic mass unit (amu) = 1822.8880 electron mass

1 Electron volt (eV) = 23.06055 kcal-mol-1

1 Hartree = 627.5095 kcal-mol-1 = 27.2114 eV

1 Bohr-electron = 2.541746 Debye = 2.541746 x 10-18 esu-cm

1 Debye2-angstrom-2-amu-1 = 42.2561 km-mol-1 = 5.82573 x 10-3 cm-2-atm-1 at STP

1 Hartree-1/2-Bohr-1-amu-1/2 = 219474.6 cm-1

Electric field: 1 au = 5.142206 x 1011 V-m-1

Electric polarizability: 1 au = 1.648777 x 10-41 C2-m2-J-1

Dipole moment = 1 Bohr-electron = 8.478352 x 10-30 C-m