diff --git a/qpms/scatsystem.h b/qpms/scatsystem.h
index 16578a6..4b0fb9f 100644
--- a/qpms/scatsystem.h
+++ b/qpms/scatsystem.h
@@ -124,8 +124,8 @@ typedef struct qpms_ss_particle_orbitinfo {
 struct qpms_trans_calculator;
 
 typedef struct qpms_scatsys_t {
-	// TODO does bspec belong here?
-	qpms_tmatrix_t **tm; ///< T-matrices in the system
+	qpms_qpms_epsmu_generator_t *medium; ///< Optical properties of the background medium.
+	qpms_abstract_tmatrix_t **tm; ///< T-matrices in the system
 	qpms_ss_tmi_t tm_count; ///< Number of all different T-matrices
 	qpms_ss_tmi_t tm_capacity; ///< Capacity of tm[].
 	qpms_particle_tid_t *p; ///< Particles.
@@ -167,6 +167,8 @@ typedef struct qpms_scatsys_t {
 	struct qpms_trans_calculator *c;
 } qpms_scatsys_t;
 
+typedef struct qpms_scatsys_at_omega_t {
+
 /// Convenience function to access pi'th particle's bspec.
 static inline const qpms_vswf_set_spec_t *qpms_ss_bspec_pi(const qpms_scatsys_t *ss, qpms_ss_pi_t pi) {
 	return ss->tm[ss->p[pi].tmatrix_id]->spec;
diff --git a/qpms/tmatrices.h b/qpms/tmatrices.h
index c208aa0..ba29e48 100644
--- a/qpms/tmatrices.h
+++ b/qpms/tmatrices.h
@@ -8,6 +8,8 @@
 #include "materials.h"
 #include <stdio.h>
 
+
+
 struct qpms_finite_group_t;
 typedef struct qpms_finite_group_t qpms_finite_group_t;
 
@@ -503,6 +505,25 @@ qpms_errno_t qpms_tmatrix_axialsym_RQ_transposed_fill(complex double *target,
 		);
 
 
+/// An "abstract" t-matrix, contains a T-matrix generator instead of actual data.
+typedef struct qpms_tmatrix_function_t {
+        /** \brief VSWF basis specification, NOT owned by qpms_tmatrix_t by default.
+         *
+         *  Usually not checked for meaningfulness by the functions (methods),
+         *  so the caller should take care that \a spec->ilist does not
+         *  contain any duplicities and that for each wave with order \a m
+         *  there is also one with order \a −m.
+         */
+        const qpms_vswf_set_spec_t *spec;
+        const qpms_tmatrix_generator_t *gen; ///< A T-matrix generator function.
+} qpms_tmatrix_function_t;
+
+/// A recepy to create another T-matrices from qpms_tmatrix_fuction_t by symmetry operations.
+typedef struct qpms_derived_tmatrix_function_t {
+	const qpms_tmatrix_function_t *t;
+}
+
+
 #if 0
 // Abstract types that describe T-matrix/particle/scatsystem symmetries
 // To be implemented later. See also the thoughts in the beginning of groups.h.