diff --git a/qpms/scatsystem.c b/qpms/scatsystem.c
index 7308cc7..e4965d0 100644
--- a/qpms/scatsystem.c
+++ b/qpms/scatsystem.c
@@ -147,8 +147,11 @@ static void add_orbit_type(qpms_scatsys_t *ss, const qpms_ss_orbit_type_t *ot_cu
 qpms_scatsys_at_omega_t *qpms_scatsys_apply_symmetry(const qpms_scatsys_t *orig,
     const qpms_finite_group_t *sym, complex double omega, 
     const qpms_tolerance_spec_t *tol) {
+  if(orig->lattice_dimension)
+    QPMS_NOT_IMPLEMENTED("Periodic systems not yet done");
   // TODO check data sanity
 
+
   qpms_l_t lMax = 0; // the overall lMax of all base specs.
   qpms_normalisation_t normalisation = QPMS_NORMALISATION_UNDEF;
 
@@ -178,6 +181,8 @@ qpms_scatsys_at_omega_t *qpms_scatsys_apply_symmetry(const qpms_scatsys_t *orig,
   // Allocate T-matrix, particle and particle orbit info arrays
   qpms_scatsys_t *ss;
   QPMS_CRASHING_MALLOC(ss, sizeof(qpms_scatsys_t));     
+  ss->lattice_dimension = orig->lattice_dimension;
+  // TODO basic periodic lattices related stuff here.
   ss->lenscale = lenscale;
   ss->sym = sym;
   ss->medium = orig->medium;
diff --git a/qpms/scatsystem.h b/qpms/scatsystem.h
index 5c74c1f..0b32ec7 100644
--- a/qpms/scatsystem.h
+++ b/qpms/scatsystem.h
@@ -135,7 +135,42 @@ typedef struct qpms_ss_derived_tmatrix_t {
 struct qpms_trans_calculator;
 struct qpms_epsmu_generator_t;
 
+/// Common "class" for system of scatterers, both periodic and non-periodic.
+/**
+ * Infinite periodic structures (those with \a lattice_dimension > 0) 
+ * have the following additional members filled:
+ *  - lattice_basis_csystem
+ *  - lattice_basis
+ *  - reciprocal_basis_csystem
+ *  - reciprocal_basis
+ *  - unitcell_volume
+ * These are ignored for finite systems (lattice_dimension == 0).
+ *
+ */
 typedef struct qpms_scatsys_t {
+	/// Number of dimensions in which the system is periodic from the range 0–3.
+	int lattice_dimension;
+	
+	/// Coordinate system for \a lattice_basis.
+	/** This is mandatory for \a lattice_dimension != 0 */
+	qpms_coord_system_t lattice_basis_csystem;
+	/// (Direct) lattice basis of the system (only \a lattice_dimension elements are used)
+	/** This is mandatory for \a lattice_dimension != 0 */
+	anycoord_point_t lattice_basis[3];
+
+	/// Coordinate system for \a reciprocal_basis.
+	qpms_coord_system_t reciprocal_basis_csystem;
+	/// Reciprocal lattice basis. 
+	/**(TODO specify whether it includes 2π or not) */
+	anycoord_point_t reciprocal_basis[3];
+
+	/// Unitcell volume (irrelevant for non-periodic systems).
+	/** The dimensionality of the volume corresponds to lattice_dimension, so
+	 * for lattice_dimension == 1, this will actually be lenght and for
+	 * lattice_dimension == 2, a 2D area.
+	 */
+	double unitcell_volume;
+
 	struct qpms_epsmu_generator_t medium; ///< Optical properties of the background medium.
 	
 	/// (Template) T-matrix functions in the system.