diff --git a/cij/core/calculator.py b/cij/core/calculator.py
index 67fd895..8f3fbd9 100644
--- a/cij/core/calculator.py
+++ b/cij/core/calculator.py
@@ -111,7 +111,7 @@ def _calculate_pressure_static(self, order: int = 3):
 
         strains = calculate_eulerian_strain(volumes[0], volumes)
         strain_array = calculate_eulerian_strain(volumes[0], self.v_array)
-        _, static_energy_array = polynomial_least_square_fitting(
+        static_energy_array = polynomial_least_square_fitting(
             strains, static_energies, strain_array,
             order=order
         )