QEC can calculate cross sections for molecule with atoms as heavy as Ar and up to 20-25 atoms in total. However if you want to go beyond Ar you can proceed with caution.
The complexity level increases for us with the number of electrons in the system. If the number of electrons in the heaviest atom in the molecule contains more than 18 electrons, then the calculation may be quite difficult and computationally demanding, if it is more than 36 it will certainly be complex and we recommend using Effective Core Potentials (ECPs) to do it with high accuracy. In absence of ECPs you can employ a special basis set to generate sensible results albeit not as accurate as with ECPs. In other words, calculations get progressively more difficult as the heaviest atom gets heavier. Zr is heavier than Cu, calculations for ZrX will be harder than CuX. And if the heaviest atom in the molecule is heavier than Kr then ECPs should be used to improve the accuracy. Luckily the ECP implementation in QEC is anticipated next year and soon this module will be available to QEC users, as well as our team will be performing consultancy calculations.
Another aspect to consider is the number of atoms in the molecule. Identifying the breakup products for neutral dissociation and dissociative ionisation becomes more difficult as the number of atoms in the molecule increases, since the number of bonds increases and in theory any of them could break. Quantemol has a method of estimating dissociative ionisation products based on machine learning, which will be integrated to Quantemol-EC as well, but currently available for paying users of QuantemolDB database. Learn more here: https://www.quantemoldb.com/q-machine-learning/