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Statistical Mechanics

NPL has been developing a new software program "StatMech" for the calculation of the thermodynamic properties of gas phase species from molecular constants using statistical mechanics. The software will handle monatomic, diatomic and polyatomic molecules with, using the current version (v1.1), up to 8 separate atoms. One aim of this work is to provide a comparison between the "best" assessed values for certain thermodynamic properties and those calculated from Statistical mechanics using molecular constants derived from ab-initio quantum mechanics. A preliminary comparison can be seen here for calculations of S298 using StatMech and a small reference collection of values deemed to be the most reliable, now also stored within the SGTE Substance database. These results can also be compared graphically.

The fundamental constants are currently taken only from the CODATA Recommended Values for the Fundamental Physical Constants: 2006 Evaluation. In due course we plan to introduce a facility for these constants to be selected by the user to allow comparison with earlier compilations of thermodynamic properties. This means that there will be a difference typically in the third decimal place when compared with the JANAF 1985 edition.

To download the software please click here. StatMech is at present a stand-alone program running under the .NET framework runtime environment (version 2 or higher is required). The software will be available to all collaborators and sympathetic users at no cost. It is likely that, in due course, it will be implemented within version 5 of MTDATA but will not require a permit file for use.

The current version allows anharmonic contributions to be taken into account for diatomic molecules but does not yet cover cases where internal rotations are hindered. The assumption is also made at present that a change in electronic state does not affect the vibrational frequencies or atomic coordinates.

Data may be input either interactively via an easy-to-use form or from a defined input file. The sort of information that is required by the program is atom positions and atomic weights or the moment of inertia, vibrational frequencies and electronic energy levels and their degeneracies. The information may be modified and saved to a file for use at a later date.

A facility has been implemented to allow viewing of the molecule defined by the three dimensional array of coordinates in order to perform a visual check on the shape of the molecule.

The output from the program is standard thermodynamic tables eg Cp, S, H-H(ref), G-H(ref). Software from the Fit and Plot module of MTDATA has been incorporated into StatMech for the automatic curve fit of the calculated heat capacity data into datasets suitable for loading directly into MTDATA databases.

Updated 30 September 2009