As for any r i system, different chromatography conditions should have negligible influence on the r i value of the analytes, suggesting that there is a correlation between the r i values and structural properties, expressed as molecular descriptors. This is the main principle used by the quantitative structure-retention relationship (QSRR) based models, enabling the construction of QSRR models that either use all or a selection of descriptors to predict r i values. However, difficulties arise when calculating descriptors due to convergence issues related to calculation time-out or local minima. Moreover, descriptors can often be difficult to interpret, since they contain mathematical representations of the molecular structure.