The logP Plugin calculates the octanol/water partition coefficient, which is used in QSAR analysis and rational drug design as a measure of molecular hydrophobicity. The calculation method is based on the publication of Viswanadhan et al. The logP of a molecule is composed of the increment of its atoms. However, the algorithm described in the paper was modified at several points: 1. Many atomic types were redefined to accommodate electron delocalization and contributions of ionic forms were added. 2. The logP of zwitterions are calculated from their logD value at their isoelectric point. 3. The effect of hydrogen bonds on the logP is considered if there is a chance to form a six membered ring between suitable donor and acceptor atoms. 4. New atom types were introduced especially for sulfur, carbon, nitrogen and metal atoms.