A crystal structure prediction study for the molecular salt pyridinium chloride was carried out with Z′ = 1 and Z′ = 2 in all 230 space groups. The predicted crystal structures were ranked by energy using dispersion-corrected density functional theory calculations, with the dispersion-correction parameterised against uncharged systems only. The experimental structures are ranked 1st and shared 2nd, which suggests that the dispersion-correction parameters are likely to be at least partially transferable to charged systems. For the structure generation step, a high-accuracy tailor-made force field was prepared for pyridinium chloride. The accuracy of the tailor-made force field is comparable to those for neutral molecules. A problem caused by spurious pseudo-symmetry due to the use of a cascade of energy potentials of different accuracies is described, as is its solution. Our calculations confirm that a previously reported P21/m structure for pyridinium chloride should have been reported in P, our calculations suggest the existence of a high-pressure polymorph in Pnma, and our calculations suggest that the high-temperature phase of pyridinium chloride might be subtly different from the high-temperature phase of pyridinium iodide.
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