diff --git a/networks/metal_chem/_parameters b/networks/metal_chem/_parameters
index 02964ce61..eb1cf6e17 100644
--- a/networks/metal_chem/_parameters
+++ b/networks/metal_chem/_parameters
@@ -9,7 +9,8 @@ redshift                          real               0e0
 metallicity                          real               1e0
 # dust to gas ratio relative to solar
 dust2gas_ratio                 real               1e0
-
+# starting index for gas-phase species (after the ices)
+idx_gas_species                int                0
 
 # Cosmic ray ionization rate (per s)
 crate                          real               0e0