Data for manuscript "NaCl nucleation from brine in seeded simulations: sources of uncertainty in rate estimates" J.Chem. Phys. 2018. This archive contains 3 directories... 1) Figure 1/ Contains the electrolyte chemical potential data sets as calculated in this work and as extracted from earlier work by some of the authors. Each data set in figure 1 is stored as a two-column ASCII text file with the molality in the first column and the corresponding electrolyte chemical potential in the second column. 2) nucleus_size_vs_time/ Contains, for each of three molalities, nucleus size vs simulated time data for a range of initial nucleus seed sizes (nsphere). Each seed nucleus size directory contains multiple sub-directories corresponding to a different way of assigning NaCl ions to clusters, identified as s_1 to s_5. Several realisations of each trajectory are provided for averaging purposes. Each realisation is stored as a seperate ASCII text file with multiple columns described by the header. 3) SM/ Additional data series used in producing the plots within the supplementary information. Formatted as two-column ASCII text files x,y.