Modern quantum chemical electronic structure methods typically applied to localized chemical bonding are developed to predict atomic structures and free energies for meso-tetraalkylporphyrin self-assem- bled monolayer (SAM) polymorph formation from organic solution on highly ordered pyrolytic graphite surfaces. Large polymorph- dependent dispersion-induced substrate−molecule interactions (e.g., −100 kcal mol−1 to −150 kcal mol−1 for tetratrisdecylporphyrin) are found to drive SAM formation, opposed nearly completely by large polymorph-dependent dispersion-induced solvent interactions (70– 110 kcal mol−1) and entropy effects (25–40 kcal mol−1 at 298 K) fa- voring dissolution. Dielectric continuum models of the solvent are used, facilitating consideration of many possible SAM polymorphs, along with quantum mechanical/molecular mechanical and disper- sion-corrected density functional theory calculations. These predict and interpret newly measured and existing high-resolution scanning tunnelling microscopy images of SAM structure, rationalizing poly- morph formation conditions. A wide range of molecular condensed matter properties at room temperature now appear suitable for pre- diction and analysis using electronic structure calculations.