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Statistical Mechanics module of MTDATA Studio

NPL has developed a new software module "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 more than 100 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.

To download the software please click here. StatMech is integrated into MTDATA Studio and will run without an MTDATA permit file under an Evaluation Licence.

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 module 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 number of example input files may be found in the folder "Work\StatMech Examples"

A facility has been implemented to allow viewing of the molecule defined by the three dimensional array of coordinates. This provides an important check on the molecular dimensions (bond lengths and angles), a basic point group symmetry check is also performed. The molecular viewer uses Jmol via an HTML form with links to the file "statmech_mol.html" which can be found in "Work\Jmol"

The output from the module 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.

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.

Updated 5 December 2011