diff --git a/fitburst/routines/derivative.py b/fitburst/routines/derivative.py
index 5d1df0e..7b63e87 100644
--- a/fitburst/routines/derivative.py
+++ b/fitburst/routines/derivative.py
@@ -733,6 +733,10 @@ def deriv_model_wrt_scattering_index(parameters: dict, model: float, component:
     num_freq, num_time, num_component = model.timediff_per_component.shape
     deriv_mod_int = np.zeros((num_freq, num_time, num_component), dtype=float)
 
+    burst_width = parameters["burst_width"][component]
+    spectral_index = parameters["spectral_index"][component]
+    spectral_running = parameters["spectral_running"][component]
+
     # now loop over each model component and compute contribution to derivative.
     for current_component in range(num_component):
         current_amplitude = model.amplitude_per_component[:, :, current_component]
@@ -744,8 +748,6 @@ def deriv_model_wrt_scattering_index(parameters: dict, model: float, component:
         scattering_timescale = parameters["scattering_timescale"][0]
         current_timediff = model.timediff_per_component[:, :, current_component]
         scat_times_freq = scattering_timescale * freq_ratio ** scattering_index
-        spectrum = 10 ** amplitude * freq_ratio ** (spectral_index + spectral_running * log_freq)
-        spectrum *= freq_ratio ** (-scattering_index)
 
         # define argument of error and scattering timescales over frequency.
         spectrum = current_amplitude * freq_ratio[:, None] ** (-scattering_index)
@@ -1649,7 +1651,7 @@ def deriv2_model_wrt_burst_width_scattering_index(parameters: dict, model: float
     # now loop over each frequency and compute mixed-derivative array per channel.
     for freq in range(current_model.shape[0]):
         freq_ratio = model.freqs[freq] / ref_freq
-        log_freq = np.log_freq_ratio
+        log_freq = np.log(freq_ratio)
         sc_time_freq = sc_time * freq_ratio ** sc_index
         spectrum = 10 ** amplitude * freq_ratio ** (spectral_index + spectral_running * np.log(freq_ratio))
         spectrum *= freq_ratio ** (-sc_index)