Hartree To kJ/mol Calculator – Energy Converter

Convert Hartree to kJ/mol or kJ/mol to Hartree using the 2022 CODATA energy constant, with results in kcal/mol, J/mol, electron volts, and joules.

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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 = rounded conversion factor (2022 CODATA central-value calculation: approximately2625.4996394792 kJ/mol)

To convert kJ/mol back to Hartree, divide by the same conversion factor. Using2625.5 instead of the 2022 central-value factor introduces approximately0.0000137% relative error. Choose rounding to suit the required precision; the calculator retains the unrounded factor.

Hartree to kJ/mol Conversion Table
Hartree (Eₕ)	kJ/mol	Context
0.0001	0.263	Below chemical accuracy
0.0015936 (approx.)	4.184 (approx.)	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 2022 CODATA value is4.3597447222060(48)×10⁻¹⁸ J. The parentheses indicate standard uncertainty in the final digits. It equals2hcR_∞ and the Coulomb energy scale for elementary charges separated by one Bohr radius. Twice the actual hydrogen ionization energy is only an approximation, owing to finite nuclear mass and other corrections.

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. Many molecular quantum-chemistry programs report electronic energies in Hartree. Always check the output units: VASP, for example, reports relevant total-energy output in eV.

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

The factor is derived from Avogadro’s constant and the SI value of the Hartree. Rounding it to2625.5 produces approximately0.0000137% relative error; whether that matters depends on the requested precision.

Quantum Chemistry Energy Unit Reference

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

Energy Equivalents for 1 Hartree (rounded 2022 CODATA)
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). Meeting this error target on an appropriate benchmark does not by itself establish reliability for a different system or reaction. 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.

Illustrative Error Magnitudes by Quantum Chemistry Method
Method	MAE (kcal/mol)	MAE (Eₕ)	Chemical Accuracy?
CCSD(T)/CBS	~0.1	~0.000159	Yes
MP2/large basis	1–5	0.0016–0.0080	Borderline
DFT hybrid (B3LYP)	3–5	0.0048–0.0080	Often not
DFT modern (ωB97M-V)	0.5–1.5	0.0008–0.0024	Near threshold
Hartree–Fock (HF)	10–50	0.016–0.080	No
Illustrative magnitudes, not universal performance guarantees: MAE depends on the benchmark, system and calculation protocol. MAE = mean absolute error; CBS = complete basis set extrapolation.