Interpolated Partition Function

To generate partition functions from an externally provided table of values (such as those provided by HITRAN) add an "InterpolatedPartitionFunction" object at the molecule level. Provided the active property is set then, when requiring partition functions, PGOPHER will interpolate from the provided table, rather than attempting a sum over all calculated levels. Unless several ground state vibrational levels are included in the calculation, this approach is likely to be necessary for full accuracy in absolute intensities. To load the table, right click on the object in the tree view and select "Import..."; a simple text file is expected with at least two values on each line, the temperature (K) and the corresponding value of the partition function (More columns can be present.) On loading the file you will normally be prompted to select the columns containing the temperature (T) and partition function values, Q. After loading, the values will be visible as extra parameters, and can be edited as with other values.

With this set up and active, the populations are taken as:

g/(Scale×Q_ext) exp( -(E-E_min-E_zero) / k_BT )

where Q_ext is the interpolated value derived from the values given. E_min is used if the AutoZero flag is on, and is the lowest calculates energy, and has the effect of shifting the energy origin of the calculated levels to zero. (The origin is otherwise dependent on how the data file is set up and the choice of the Hamiltonian.) E_zero is the origin used when the external partition function was calculated.