diff --git a/include/poisson.h b/include/poisson.h
new file mode 100644
index 0000000000000000000000000000000000000000..f19c2ac75e24d475d55b4491034625ead869b79e
--- /dev/null
+++ b/include/poisson.h
@@ -0,0 +1,76 @@
+/** @file poisson.h
+ *  @brief Structures and functions for the Poisson solver
+ */
+
+#ifndef _POISSON_H_
+
+#include "cstream.h"
+#include "rz_array.h"
+
+#ifndef _GRID_H_
+#include "grid.h"
+#endif
+
+typedef struct pois_grid_t pois_grid_t;
+
+struct pois_grid_t {
+  RECT_COORDS;
+  LEAF_FIELDS(pois_grid_t);
+  int ext_bound;
+
+  rz_array_t *phi;
+  rz_array_t *charge;
+  rz_array_t *error;
+};
+
+/*!< With this structure we define a "problem" for the Poisson/Helmholtz solver.
+ * Thus we differentiate between the electrostatic (Poisson) or the
+ * photo-ionization (Helmholtz) uses.
+ */
+typedef struct pois_problem_t pois_problem_t;
+
+struct pois_problem_t {
+  int max_level;
+  int extra_levels;
+  double max_error;
+
+  int bnd_right;
+  int bnd_top;
+  int bnd_bottom;
+};
+
+typedef struct pois_boundaries_t {
+  REAL *left, *right, *top, *bottom;
+  int r, z;
+} pois_boundaries_t;
+
+/*!< Note that the electric fields are computed here as the derivatives
+ * of \f$\phi\f$, and not _minus_ the derivatives.
+ *
+ * This is because our \f$\phi\f$ is not actually the electrostatic potential
+ * in its standard definition but its opposite. This simplifies the computations
+ * since we can use the charge as the source of the Poisson equation.
+ */
+#define UNCHECK_ER_RZ(grid_, ir_, iz_)					\
+  (((RZ(grid_->phi, (ir_) + 1, iz_) - RZ(grid_->phi, ir_, iz_))		\
+    / dr[grid_->level]))
+
+#define UNCHECK_EZ_RZ(grid_, ir_, iz_)					\
+  (((RZ(grid_->phi, ir_, (iz_) + 1) - RZ(grid_->phi, ir_, iz_))	\
+    / dz[grid_->level]))
+
+/*!< The easiest way (and not too performance costly) is to check the
+ *  boundaries where the field can be calculated.
+ *
+ *  Note that, if everything is OK, the 0 there that we set should never be
+ *  used again (and hence any number would be ok (use NaN for debugging) */
+#define ER_RZ(grid_, ir_, iz_)			\
+  (((ir_) < ((grid_)->r1 + 1) && ((ir_) > ((grid_)->r0 - 2)))? \
+   UNCHECK_ER_RZ(grid_, ir_, iz_): 0)
+
+#define EZ_RZ(grid_, ir_, iz_)			\
+  (((iz_) < ((grid_)->z1 + 1) && ((iz_) > ((grid_)->z0 - 2)))? \
+   UNCHECK_EZ_RZ(grid_, ir_, iz_): 0)
+
+#define _POISSON_H_
+#endif