NPL has been developing a new software program
"StatMech" for the calculation of the thermodynamic properties of gas
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
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
To download the software please click here.
StatMech is at present a stand-alone program running under
framework runtime environment (version 2 or higher is
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.
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.
30 September 2009