diff --git a/vignettes/si.Rmd b/vignettes/si.Rmd
index bf51050..66f58db 100644
--- a/vignettes/si.Rmd
+++ b/vignettes/si.Rmd
@@ -112,4 +112,25 @@ head(od_sim, 2)
 
 Note that the output has duplicate columns: `si_to_od()` joins data from the origin and destination objects into the resulting OD object.
 
+# Running an unconstrained SIM
+
+A simplistic SIM can be created just based on the distance between points:
+
+```{r unconstrained1}
+si_power = function(od, beta) {
+  (od[["distance_euclidean"]] / 1000)^beta
+} 
+od_model = si_model(od_sim, fun = si_power, beta = -0.8)
+plot(od_model["res"], logz = TRUE)
+```
+
+This approach, ignoring all variables at the level of trip origins and destinations, results in flow estimates with no units.
+Before learning how to run constrained SIMs in the next section, let's add estimates of each zone's population to the model:
+
+```{r}
+# Todo
+```
+
+
+
 # References