Predetermination of relevant parameters for QCxMS (x = EI or CID) simulations

[ghost]

As stated in the documentation (https://xtb-docs.readthedocs.io/en/latest/qcxms_doc/qcxms_run.html#ei-method-specific-keywords and https://xtb-docs.readthedocs.io/en/latest/qcxms_doc/qcxms_run.html#cid-method-specific-keywords), there are parameters that have significant influence on the fragmentation pattern of the molecular ion and therefore, the resulting mass spectrum. I reckon that
For EI, the important set of parameters is [eimp0, eimpw, ieeatm, tmax]
For CID (for fullauto mode), the important set of parameters is [elab, pgas, lchamb, tmax]
Can certain simulations be run to obtain a (good) initial guess of these sets ?

Furthermore, I have the following questions:

1. Is the value for eimpw the most probable impact excess energy per atom of the corresponding distribution ?
2. How will I know if the simulation fails because of the type of IEE distribution ?
3. How should the limits for the distribution be set ?

[JayTheDog]

Overall, the default setting that are listed in the docs that you have provided are those that were found to be the best initial guess for most systems. This refers to a majority of molecules, but does not mean that it is the best for particular cases. This means you do not have to figure out all of the setting by your own, the defaults will take care of most cases!

If you want to customize your calculations, it depends on your system to which you are comparing. Take a look at the molecular ion and change the values correspondingly. Too large M+ / [M+H]+ peaks means you have to increase the internal energy.

• For CID, use fullauto and adapt the elab to find the best value.
• For EI, you could increase eimp0 and ieeatm (the latter in small steps).

For CID, the nature of the instruments that are used have a significant influence on the outcome, so its not possible to find a overall best initial guess for the set. The fullauto method tries to calculate the number of of collisions dependend on pgas and lchamb, but you can also set these manually in the collauto runmode with setcoll.

For the other questions:

1. For eimpw, the default is the best value. This is the energy distribution width, meaning a Gaussian distribution at the energy impact site. This value is not that significant, wouldn't change.
2. The computation should not fail because of the type of IEE distribution, you are only getting different kinds of results, i.e. signal intensities. Poisson showed better results (thus it's the default). Wouldn't change either.
3. Read the literature https://onlinelibrary.wiley.com/doi/10.1002/anie.201300158 (especially the supporting information)