The SAMPL Challenges aim to focus the biomolecular and physical modeling community on issues that limit the 18 accuracy of predictive modeling of protein-ligand binding for rational drug design. In the SAMPL5 log D Challenge, designed to 19 benchmark the accuracy of methods for predicting drug-like small molecule transfer free energies from aqueous to nonpolar 20 phases, participants found it difficult to make accurate predictions due to the complexity of protonation state issues. In the 21 SAMPL6 log P Challenge, we asked participants to make blind predictions of the octanol-water partition coefficients of neutral 22 species of 11 compounds and assessed how well these methods performed absent the complication of protonation state 23 effects. This challenge builds on the SAMPL6 pK a Challenge, which asked participants to predict pK a values of a superset of the 24 compounds considered in this log P challenge. Blind prediction sets of 91 prediction methods were collected from 27 research 25 groups, spanning a variety of quantum mechanics (QM) or molecular mechanics (MM)-based physical methods, knowledge-based 26 empirical methods, and mixed approaches. There was a 50% increase in the number of participating groups and a 20% increase 27 in the number of submissions compared to the SAMPL5 log D Challenge. Overall, the accuracy of octanol-water log P predictions 28 in SAMPL6 Challenge was higher than cyclohexane-water log D predictions in SAMPL5, likely because modeling only the neutral 29 species was necessary for log P and several categories of method benefited from the vast amounts of experimental octanol-water 30 log P data. There were many highly accurate methods: 10 diverse methods achieved RMSE less than 0.5 log P units. These included 31 QM-based methods, empirical methods, and mixed methods with physical modeling supported with empirical corrections. A 32 comparison of physical modeling methods showed that QM-based methods outperformed MM-based methods. The average 33 RMSE of the most accurate five MM-based, QM-based, empirical, and mixed approach methods based on RMSE were 0.92±0.13, 34 0.48±0.06, 0.47±0.05, and 0.50±0.06, respectively. 35 36 0.1 Keywords 37 octanol-water partition coefficient ⋅ log P ⋅ blind prediction challenge ⋅ SAMPL ⋅ free energy calculations ⋅ solvation modeling 38 1 of 50 0.2 Abbreviations 39 SAMPL Statistical Assessment of the Modeling of Proteins and Ligands 40 log P log 10 of the organic solvent-water partition coefficient ( ) of neutral species 41 log D log 10 of organic solvent-water distribution coefficient ( ) 42 pK a −log 10 of the acid dissociation equilibrium constant 43 SEM Standard error of the mean 44 RMSE Root mean squared error 45 MAE Mean absolute error 46 Kendall's rank correlation coefficient (Tau) 47 R 2 Coefficient of determination (R-Squared) 48 QM Quantum Mechanics 49 MM Molecular Mechanics 50 1 Introduction 51 The development of computational biomolecular modeling methodolgoies is motivated by the goal of enabling quantitative 52 molecular design, pred...