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Overview
NIST provides several scientific databases. REST API availability varies by dataset.
1. NIST CODATA Fundamental Physical Constants
Base URL
https://physics.nist.gov/cgi-bin/cuu
No formal REST API. Data is served via CGI scripts returning HTML. The constants can be accessed programmatically via structured URLs but responses are HTML, not JSON/XML.
Workaround — machine-readable ASCII:
https://physics.nist.gov/cuu/Constants/Table/allascii.txt
Returns a fixed-width ASCII table (with header lines), not a tab-delimited file, of all fundamental constants with values, uncertainties, and units.
Individual constant lookup:
https://physics.nist.gov/cgi-bin/cuu/Value?{constant_key}
Example keys: bohrrada0 (Bohr radius), c (speed of light), h (Planck constant), e (electron charge), me (electron mass), na (Avogadro number), k (Boltzmann constant).
Example:
https://physics.nist.gov/cgi-bin/cuu/Value?h
Returns HTML page. Parse the value from the page content.
No API key required. No rate limits documented.
2. NIST Atomic Spectra Database (ASD)
Base URL
https://physics.nist.gov/cgi-bin/ASD
No formal REST API. Queries are CGI-based, returning HTML. However, machine-readable output is available via specific parameters.
Spectral lines query:
https://physics.nist.gov/cgi-bin/ASD/lines1.pl?spectra={element}&low_w={min_wavelength}&upp_w={max_wavelength}&unit={unit}&format={format}
| Parameter | Type | Description |
|---|---|---|
spectra |
string | Element symbol or ion (e.g., H, Fe, He+I, O+II). |
low_w |
float | Lower wavelength bound. |
upp_w |
float | Upper wavelength bound. |
unit |
int | 0 = Angstroms, 1 = nm, 2 = um. |
format |
int | 0 = HTML, 1 = ASCII, 2 = CSV, 3 = tab-delimited. |
line_out |
int | 0 = all, 1 = only observed, 2 = only Ritz. |
show_obs_wl |
int | 1 = show observed wavelengths. |
show_calc_wl |
int | 1 = show Ritz wavelengths. |
A_out |
int | 1 = include transition probabilities. |
Example — Hydrogen lines 3000-7000 Angstroms as CSV:
https://physics.nist.gov/cgi-bin/ASD/lines1.pl?spectra=H&low_w=3000&upp_w=7000&unit=0&format=2&line_out=0&show_obs_wl=1&A_out=1
Energy levels query:
https://physics.nist.gov/cgi-bin/ASD/energy1.pl?spectrum={element}&units={units}&format={format}
No API key required. No formal rate limits but automated bulk queries are discouraged.
3. NIST Chemistry WebBook
Base URL
https://webbook.nist.gov/cgi/cbook.cgi
No formal REST API. CGI-based with HTML output. Structured URLs can be used.
Search by name:
https://webbook.nist.gov/cgi/cbook.cgi?Name={compound}&Units=SI
Search by CAS number:
https://webbook.nist.gov/cgi/cbook.cgi?ID={cas_number}&Units=SI
Search by formula:
https://webbook.nist.gov/cgi/cbook.cgi?Formula={formula}&Units=SI
JCAMP-DX spectra (machine-readable):
https://webbook.nist.gov/cgi/cbook.cgi?JCAMP=C{cas_no_dashes}&Index=0&Type=IR
Choose a spectrum index from the compound page; not every compound has an IR spectrum. The water request ?JCAMP=C7732185&Index=0&Type=IR returned JCAMP-DX in the 2026-09-30 public probe.
Summary
NIST databases generally do not offer modern REST/JSON APIs. Data access is primarily through CGI endpoints returning HTML or delimited text. For programmatic use, the ASCII/CSV output options from ASD are the most practical. No authentication is required for any NIST endpoint.