Tools · Fluids & Thermodynamics

Transition-State-Theory Rate Calculator

Turn computed activation free energies, or reactant and transition-state thermochemistry, into rate constants over temperature, with Arrhenius fits.

Updated October 1, 2026

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The Transition-State-Theory Rate Calculator connects quantum chemistry to kinetics. Give it activation energies (or the thermochemistry of reactants and transition states) and it computes rate constants k(T) over a temperature range with canonical Eyring transition-state theory, then fits Arrhenius parameters you can use in a kinetic model. It runs on CPU.

How it works

For each reaction and temperature:

k(T) = σ · κ(T) · (k_B·T / h) · (c°)^(1−m) · exp(−ΔG‡°(T) / RT)

m, the molecularity (1 to 3), and c°, the standard-state concentration: standard_state is 1bar for gas-phase or 1M for solution. ΔG‡ must already refer to that standard state; the tool doesn't convert between them.
σ, the reaction-path degeneracy (default 1). Don't count symmetry twice if your ΔG‡ already includes it.
κ, tunneling: 1, or the Wigner correction from the imaginary frequency with tunneling: wigner. Wigner is a leading-order correction, so the diagnostics warn when κ exceeds 1.5 or the temperature falls below the crossover temperature, where it stops being reliable.

It then fits both the two-parameter Arrhenius form A·exp(−Ea/RT) and the modified form A·Tⁿ·exp(−Ea/RT) to the computed rate constants.

Inputs

reactions (up to 200), each one of:
- an activation row: dg_act, or dh_act and ds_act, plus molecularity;
- a species row: reactants and ts names, looked up in species_thermo, so that ΔG‡(T) = G(TS) − ΣG(reactants).
- Optional sigma, and imag_freq_cm1 (required for Wigner tunneling).
species_thermo (for species rows): G, or H and S, for each species, at one temperature or several (interpolated; temperatures outside the table are rejected rather than extrapolated). Absolute energies such as hartrees are fine, since only differences matter.
temperatures: a range (t_min_k, t_max_k, n_points, spaced evenly in 1/T by default) or a list, 3 to 500 points between 10 and 5,000 K.
Units: energy_unit (kj_mol, kcal_mol, j_mol, ev, hartree), entropy_unit (j_mol_k or cal_mol_k), and concentration_unit, the basis for the rate constants of bimolecular and termolecular steps.

Outputs

File	Contents
`rate_constants.csv`	k and log₁₀ k at every temperature, with units, κ, and the activation parameters used
`arrhenius_fit.csv`	Both fitted forms per reaction: A, n, Ea, the temperature range, and the fit quality
`diagnostics.json`	Constants, assumptions, and warnings such as negative barriers, termolecular steps, or a poor fit

Kinetic Parameter Fitter for rate constants from experimental data; Cantera to use them in a simulation; PySCF Electronic Structure for quantum-chemistry energies.

Run it from the API

Submit with Submit a job and the job_type below. Price it first with Estimate job reservation cost: submitting reserves that amount from your wallet, and the charge settles at the actual runtime.