diff --git a/qpms/scatsystem.h b/qpms/scatsystem.h
new file mode 100644
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--- /dev/null
+++ b/qpms/scatsystem.h
@@ -0,0 +1,34 @@
+/*! \file scatsystem.h
+ * \brief Modern interface for finite lattice calculations, including symmetries.
+ */
+#ifndef QPMS_SCATSYSTEM_H
+#define QPMS_SCATSYSTEM_H
+
+/// A T-matrix.
+/** In the future, I might rather use a more abstract approach in which T-matrix
+ *  is a mapping (function) of the field expansion coefficients.
+ *  So the interface might change.
+ *  For now, let me stick to the square dense matrix representation.
+ */
+typedef struct qpms_tmatrix_t {
+	const qpms_vswf_set_spec_t *spec; ///< NOT owned by qpms_tmatrix_t by default.
+	complex double *m; ///< Matrix elements 
+} qpms_tmatrix_t;
+/// Returns a pointer to the beginning of the T-matrix row \a rowno.
+static inline complex double *qpms_tmatrix_row(qpms_tmatrix_t *t, size_t rowno){
+	return t->m + (t->spec->n * rowno);
+}
+/// NOT IMPLEMENTED Initialises a zero T-matrix.
+qpms_tmatrix_t *qpms_tmatrix_init(const qpms_vswf_set_spec_t *bspec);
+/// NOT IMPLEMENTED Destroys a T-matrix.
+void qpms_tmatrix_free(qpms_tmatrix_t *t);
+
+
+/// A particle, defined by its T-matrix and position.
+typedef struct qpms_particle_t {
+	// Does it make sense to ever use other than cartesian coords for this?
+	cart3_t pos; ///< Particle position in cartesian coordinates.
+	const qpms_tmatrix_t *tmatrix; ///< T-matrix; not owned by qpms_particle_t.
+} qpms_particle_t;
+
+#endif //QPMS_SCATSYSTEM_H