diff --git a/binarycpython/utils/grid_options_defaults.py b/binarycpython/utils/grid_options_defaults.py
index c994fae9dc4b3290103b0503bf9d72c92a02fbf5..707e0714d957cf19d532a3c9770d2c95c021969c 100644
--- a/binarycpython/utils/grid_options_defaults.py
+++ b/binarycpython/utils/grid_options_defaults.py
@@ -45,6 +45,7 @@ grid_options_defaults_dict = {
     "run_zero_probability_system": True, # Whether to run the zero probability systems
     "_zero_prob_stars_skipped": 0,
     "ensemble_factor_in_probability_weighted_mass": False, # Whether to multiply the ensemble results by 1/probability_weighted_mass
+    "multiplicity_fraction_function": 0, # Which multiplicity fraction function to use. 0: None, 1: Arenou 2010, 2: Rhagavan 2010, 3: M&S 2017
     ##########################
     # Execution log:
     ##########################
@@ -487,6 +488,7 @@ grid_options_descriptions = {
     "run_zero_probability_system": "Whether to run the zero probability systems. Default: True. Input: boolean",
     "_zero_prob_stars_skipped": "Internal counter to track how many systems are skipped because they have 0 probability",
     "ensemble_factor_in_probability_weighted_mass": "Flag to multiply all the ensemble results with 1/probability_weighted_mass",
+    "multiplicity_fraction_function": "Which multiplicity fraction function to use. 0: None, 1: Arenou 2010, 2: Rhagavan 2010, 3: M&S 2017",
 }
 
 ###