Frequently Asked Questions/Notes on common error messages

• Try the button - your spectrum may simply not be in the current plot range.
• The transition moment between the states of interest may not be correct; check that one is present, and that the component or axis is set to give a transition between the states involved.
• See also "Overflow in Boltzmann" below.
  

If the plotted spectrum shows with no net width to lines, either because you have not chosen to set one with "Gau" or "Lor", or because they are rather smaller than the effective plot resolution, then the resulting plot can easily be misleading. This is because of issues with closely spaced transitions - do they combine as one strong line or several weak ones? A physically meaningful plot is only guaranteed if the resolution of the plot, equal to (Fmax - Fmin)/nDF, is significantly less (by a factor of 3 or more) than the linewidth used for the simulation.

No error message, but the quantum numbers are not as expected

All quantum numbers shown, apart from total angular momentum and symmetry, can give strange computed values, particularly in the presence of state mixing, but also for certain combinations of parameter values and quantum numbers. The algorithm used by PGOPHER to determine quantum numbers is affected by the EigenSearch and LimitSearch settings in the Manifold. For linear molecules, see also the OmegaOrder setting.

Problem loading a .pgo or .ovr file

The PGOPHER native format .pgo and .ovr files must have the right extension to load correctly; when downloading from a web page or from an email the extension can sometimes be changed to .xml, or .xml is added to the filename. To fix this, simply rename the file to have the correct extension. (Versions of PGOPHER after 6.0.163 will load files that have been renamed in this way automatically.) If this is not the problem, both types of files are XML text files, and can be edited with most text editors.

All of these can result from a large negative energy causing the Boltzmann expression exp(-E/kT) to overflow, or to give an unphysical population for some high J levels. See the discussion of J range and Partition Functions for a discussion of the causes of this problem and possible solutions. Very low temperatures can also trigger this.

Progress dialog stuck on Partition Function sum

This can indicate too many levels included in the partition function, possibly because the parameters correspond to levels much too close together. See the discussion of J range and Partition Functions for a discussion of the causes of this problem and possible solutions.

Did you remember to set the right spin for all states? This results if, for example you have the changed the ground state to doublet and left the excited state as singlet (the default).

The error message means that you have asked PGOPHER to calculate an impossible level, such as a J = 1/2 level for a triplet state. It typically arises because you have not set up the spin in all the states correctly.

When least squares fitting, this normally means that you have floated a parameter (or some combination of parameters) that is not determined from the supplied experimental data. For example, you may well see this if you attempt to float both the upper and lower state origin for a transition, as only the difference is determined. To fix this, float fewer (or at least different) parameters.

This error occurs when two states with the same J and symmetry but with different statistical weights are included in the same Hamiltonian matrix, or connected by a transition moment that does not involve spin.

• It may simply indicate a strictly forbidden transition, such as an electric dipole g - g transition; check the gerade setting for the states involved.
  
• For linear molecules, this will currently occur if gerade and ungerade states in linear molecules are included in the same manifold, as PGOPHER does not currently implement this. Such states can't mix anyway, so move them to a separate manifolds, at a minimum one for g states and one for u states.
• For asymmetric tops this indicates that the eeWt, eoWt ... settings are inconsistent with the choice of symmetries in one or more of the states. (Setting all four to the same value will stop the error message, though this is (probably) not right if you have equivalent nuclei.) Check the PointGroup, Representation, C2zAxis and C2xAxis settings and see see Axis systems and symmetries for asymmetric tops.

No error message, but (unwanted) negative transition frequencies

This can arise if Initial is set false for a manifold. Setting Initial false is designed to handle spectra where the frequency scale is specified as an offset from an unknown centre frequency, but can give incorrect results in other circumstances. See also the AbsoluteE setting for the molecule.