We report a simplification of the Landau theory, permitting more direct prediction of compatible space groups in second-order phase transitions. The theory is applied to the ${\mathrm{V}}_{3}$Si transition, to predict that the lower symmetry space group is either ${{C}_{4v}}^{7}\ensuremath{-}P4\mathrm{mc}$ or ${{D}_{2d}}^{5}\ensuremath{-}P\overline{4}m2$. The former possibility admits a polar axis.