diff --git a/Simulation.ml b/Simulation.ml
new file mode 100644
index 0000000..d979da5
--- /dev/null
+++ b/Simulation.ml
@@ -0,0 +1,28 @@
+type t = {
+  basis : Basis.t;
+  nuclei : Nuclei.t;
+  overlap : Overlap.t lazy_t;
+  eN_ints : NucInt.t lazy_t;
+  kin_ints : KinInt.t lazy_t;
+  ee_ints : ERI.t lazy_t;
+}
+
+let make ~nuclei ~basis =
+  {
+    basis ; nuclei ;
+    overlap  = lazy (Overlap.of_basis basis);
+    eN_ints  = lazy (NucInt.of_basis_nuclei basis nuclei);
+    kin_ints = lazy (KinInt.of_basis basis);
+    ee_ints  = lazy (ERI.of_basis basis);
+  }
+
+
+let of_filenames ~basis ~nuclei =
+  let nuclei =
+    Nuclei.of_filename ~filename:nuclei
+  in
+  let basis =
+    Basis.of_nuclei_and_basis_filename ~nuclei ~filename:basis
+  in
+  make ~nuclei ~basis 
+