About this service

A free calculator for pure-component physical properties, estimated from molecular structure alone by the Rarey-Nannoolal group-contribution methods. Type a SMILES string or a compound name, and it returns the normal boiling point, vapour pressure, saturated liquid viscosity, thermal conductivity, surface tension and the critical properties, together with the group assignment and the arithmetic behind every number.

What it is for

Group contribution answers the question "roughly what should this substance do?" when no measurement exists — for screening candidate solvents, sanity-checking a data point, filling a gap in a simulation, or teaching. It is not a substitute for measurement, and this site is deliberately explicit about where its numbers come from and where they stop being trustworthy.

Why every number shows its working

Each result can be expanded into the group assignment, the second-order corrections, the group-interaction term and the equation with the numbers substituted. An estimate you cannot audit is hard to trust, and the breakdown also makes it obvious when a molecule has been fragmented in a way you would not have chosen.

Range of validity

These methods have limits, and the site enforces them rather than hiding them. A value is withheld where the underlying correlation is known to break down, and flagged where it is degrading. Two effects dominate:

Temperature. The temperature-dependent methods cannot follow a liquid property into the critical region — and liquid viscosity fails in the other direction, far below its reference temperature. The limits are expressed in reduced temperature T/T_c and were measured against experimental data, not assumed.
Molecule size. Group contribution assumes a molecule is the sum of its parts. Below about five heavy atoms nearly every atom is a neighbour of every other, so the groups interact instead of adding; the deviations there are several times the typical value. For such compounds a measured value almost always exists and should be preferred.

Where either applies, the reason appears in a "Range of validity" panel with the result.

Accuracy

Typical deviations from experiment, for molecules inside the ranges above, are a few percent — around 3 % for the normal boiling point, and single-digit percentages for the temperature-dependent properties when a reliable boiling point is available. The boiling point is the reference temperature for the vapour pressure, viscosity, thermal conductivity and surface tension methods, so its error propagates into all of them: if you have an experimental boiling point, the estimates built on it are markedly better than the fully predictive ones shown here.

Implementation

An independent reimplementation on RDKit by Rareytec Co., Ltd., validated against the original implementations of the methods. The structure fragmentation, the parameter tables and the equations were each checked separately; the published worked examples are reproduced digit for digit.

The surface tension method was never published. It is used here as "based on E. Olivier (2011)"; the alcohol and acid groups it does not cover were regressed by Rareytec Co., Ltd. on the openly licensed NIST/TRC ThermoML archive and are labelled as such wherever they contribute to a result.

Who provides it

This service is provided by Rareytec Co., Ltd., which works on thermophysical property data, estimation methods and laboratory measurement.

Rareytec Co., Ltd.
251 M.1 Dan Kwian, Chok Chai,
Nakhon Ratchasima 30190, Thailand
Company registration number 0305564003729 (Thailand)
President and owner: Rungarun Rarey — responsible for the content of this site.
https://www.rareytec.com

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