The NIST Physics Program produces the most comprehensive set of reliable physical reference data available anywhere.

The NIST collection of atomic energy levels, transition probabilities, and collision data is widely used by groups for characterizing and modeling all types of gaseous systems, including plasmas, planetary atmospheres, and astrophysical media, and for health physics applications. Databases and publications make these data easy to find and easy to use. The physics online databases have grown into an extensive listing. The NIST Chemistry WebBook now contains electronic and vibrational spectra for over 5000 compounds. The WebBook also contains constants of diatomic molecules.

The SRD Group has also produced several important compilations of molecular data.

The codes in the list below have the following meanings:

PC	PC product, most available for purchase, some are free
Online	Free online system
DG	Inclusion in the NIST Data Gateway - a portal providing access to many NIST scientific and technical databases. It is searchable by keyword, property, or substance name.
*	Product contains data that have undergone rigorous critical evaluation by experienced researchers who recommend best values.
Online Subsciption	Product is a yearly subscription web product.

Physics Databases

NIST Chemistry WebBook, Online, DG *
NIST Elastic-Electron-Scattering Cross-Section Database, PC, DG*
NIST Electron and Positron Stopping Powers of Materials, PC, DG*
NIST Electron Effective-Attenuation-Length Database, PC, DG *
NIST Electron Inelastic-Mean-Free-Path Database, PC, DG *
Elemental Data Index, Online, DG*
Electron-Impact Ionization Cross Section Database, Online, DG *
Handbook of Basic Atomic Spectroscopic Data Online, DG *

FLYCHK Collisional-Radiative Code, Online,DG *
SAHA Plasma Population Kinetics Database,Online,DG *
NLTE4 Plasma Population Kinetics Database,Onlin, DG *

Atomic Spectroscopic Data
Spectral Data for the Chandra X-ray Observatory, Online, DG*
Atomic Reference Data for Electronic Structure Calculation Online, DG *
Atomic Spectra Database [Version 3.0], Online, DG*
Atomic Spectral Line Broadening Bibliographic Database, Online, DG*
Atomic Transition Probability Bibliographic Database, Online, DG*
Ground Levels and Ionization Energies for the Neutral Atoms,Online, DG*
Atlas of the Spectrum of Platinum/Neon Hollow-Cathode Lamp, Online, DG*
X-Ray Transition Energies Database, Online, DG *

Fundamental Physical Constants, Online, DG*
International System of Units (SI), Online, DG*
Guidelines for Evaluating and Expressing Measurement Uncertainty, Online, DG*
Searchable Bibliography on the Constants, Online, DG*