Hartree To Kj/Mol Calculator

Reviewed By: Patrick Myers (BSME)

Enter an energy value in Hartree or kJ/mol to convert between the two. The calculator also converts to eV, J/mol, and Cal/mol using the Hartree as the common base unit.

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Hartree to kJ/mol Formula

The following formula converts energy from Hartree to kJ/mol:

E_{kJ/mol} = E_{Hartree} \times 2625.5

Variables:

E_kJ/mol = energy in kilojoules per mole
E_Hartree = energy in Hartree
2625.5 = conversion factor (CODATA 2018 precise value: 2625.4996394799 kJ/mol)

To convert kJ/mol back to Hartree, divide by 2625.5. The rounded constant introduces an error of 0.00014%, negligible for any practical electronic structure calculation.

Hartree to kJ/mol Conversion Table
Hartree (Eₕ)	kJ/mol	Context
0.0001	0.263	Below chemical accuracy
0.00159	4.184	Chemical accuracy threshold (1 kcal/mol)
0.01	26.3	Small activation barrier
0.05	131.3	Moderate activation energy
0.10	262.6	Strong activation barrier
0.16	420.1	Approx. C–H bond dissociation energy
0.174	456.8	H–H bond dissociation energy
0.50	1312.8	Half a Hartree
1.00	2625.5	1 Hartree (base value)
5.00	13127.5
10.00	26255.0
Constant: 1 Hartree = 2625.5 kJ/mol. BDE values are approximate.

What is a Hartree?

The Hartree (symbol: E_h) is the atomic unit of energy. Its 2018 CODATA value is 4.3597447222071(85) × 10⁻¹⁸ J per molecule. Physically, it equals twice the ionization energy of a ground-state hydrogen atom and matches the Coulomb interaction between two electrons separated by one Bohr radius (a₀ = 0.52918 Å).

Douglas Hartree (1897–1958), a British mathematician and physicist, introduced the atomic unit system to eliminate physical constants from Schrödinger’s equation by setting m_e = e = ℏ = 1. In atomic units, the Hartree is the natural energy scale the same way the meter is the natural length scale in SI. Gaussian, ORCA, Q-Chem, Molpro, GAMESS, VASP, NWChem, and Psi4 all report total electronic energies in Hartree by default.

Why the Conversion Factor is 2625.5

The factor comes from multiplying the per-molecule Joule value by Avogadro’s number, then dividing by 1000:

4.3597447 × 10⁻¹⁸ J × 6.02214076 × 10²³ mol⁻¹ ÷ 1000 = 2625.4996 kJ/mol

It is a derived result from Avogadro’s number and the SI value of the Hartree, not an arbitrary constant. The rounded value (2625.5) introduces 0.00014% error, far below the precision of any practical calculation.

Quantum Chemistry Energy Unit Reference

The table below shows the exact equivalents for 1 Hartree in all energy units commonly encountered in computational chemistry output files and published results:

Energy Equivalents for 1 Hartree (CODATA 2018)
Unit	Value	Used In
kJ/mol	2625.4996	Thermochemistry, IUPAC standard
kcal/mol	627.5095	US computational chemistry literature
eV	27.2114	Solid-state physics, band gaps, ionization energies
cm⁻¹ (wavenumbers)	219,474.6	Vibrational spectroscopy, IR/Raman
K (k_BT)	315,775	Statistical thermodynamics, Boltzmann populations
J (per molecule)	4.35974 × 10⁻¹⁸	SI base; rarely used directly in QC

Chemical Accuracy in Hartree

“Chemical accuracy” is the benchmark of 1 kcal/mol (4.184 kJ/mol = 0.001594 E_h). Below this threshold, computed reaction energies and activation barriers are considered reliable for experimental prediction. Because quantum chemistry codes report energies to 6+ decimal places in Hartree, small numerical differences carry large physical consequences: 0.003 E_h = 7.9 kJ/mol, enough to change a reaction from exothermic to endothermic.