Romgruppe

Romgruppe is the Norwegian term for a space group, a mathematical description of the full symmetry of a crystalline solid. In crystallography a space group combines the point symmetries of a motif (rotations, reflections, inversions) with the translational symmetry of a lattice, and includes operations unique to crystals such as screw axes and glide planes.

Space groups are classified into 230 distinct types in three-dimensional Euclidean space. They are arranged by seven crystal systems and 14 Bravais lattices, and can be denoted using standard notations such as Hermann–Mauguin (international) or Schoenflies. Each romgruppe defines the allowed arrangements of atoms in a periodic structure and constrains the possible atomic positions, site symmetries and degeneracies.

The concept originated in the late 19th and early 20th centuries as crystallographers formalized the symmetries observed in diffraction experiments; authoritative compilations are found in the International Tables for Crystallography. Determination of a romgruppe for a material is a central step in structure solution from X-ray, neutron or electron diffraction data.

Knowledge of a romgruppe is essential for predicting and interpreting physical properties that depend on symmetry, including optical activity, piezoelectricity, selection rules for spectroscopy, phonon modes and electronic band structure. In a broader, non-technical usage, romgruppe may also refer to a group or suite of rooms in architectural contexts, but its primary scientific usage denotes the symmetry group of a crystal.