diff --git a/tardis/montecarlo/montecarlo_numba/calculate_distances.py b/tardis/montecarlo/montecarlo_numba/calculate_distances.py
index 44f35a506e9..1ff8972a152 100644
--- a/tardis/montecarlo/montecarlo_numba/calculate_distances.py
+++ b/tardis/montecarlo/montecarlo_numba/calculate_distances.py
@@ -18,6 +18,11 @@
 from tardis.montecarlo.montecarlo_numba.r_packet import (
     print_r_packet_properties,
 )
+from tardis.montecarlo.montecarlo_numba.nonhomologous_grid import (
+    velocity,
+    dv_dr,
+    roots,
+)
 
 
 @njit(**njit_dict_no_parallel)
@@ -145,3 +150,27 @@ def calculate_distance_electron(electron_density, tau_event):
     """
     # add full_relativity here
     return tau_event / (electron_density * SIGMA_THOMSON)
+
+
+
+@njit(**njit_dict_no_parallel)
+def calculate_distance_line_nonhomologous(
+    r_packet, comov_nu, is_last_line, nu_line, time_explosion, numba_model
+):
+    dvdr = dv_dr(r_packet, numba_model)
+    nu = r_packet.nu
+    nu_ratio = (nu - nu_line)/nu
+    r_i = r_packet.r
+    mu_i = r_packet.mu
+    v_i = velocity(r_packet, numba_model)
+
+    #coefficients for the quartic equation
+    a = dvdr**2
+    b = -2 * C_SPEED_OF_LIGHT * nu_ratio * dvdr
+    c = C_SPEED_OF_LIGHT**2 * nu_ratio**2 + dvdr**2 * r_i**2 * (1 - mu_i**2) - (v_i - dvdr * r_i)**2
+    d = -2 * C_SPEED_OF_LIGHT * nu_ratio * dvdr * r_i**2 * (1 - mu_i**2)
+    e = C_SPEED_OF_LIGHT**2 * nu_ratio**2 * r_i**2 * (1 - mu_i**2)
+    correction = r_i * mu_i
+    possible_l = roots(a, b, c, d, e, correction)
+    distance_line = min(possible_l)
+    return distance_line
diff --git a/tardis/montecarlo/montecarlo_numba/nonhomologous_grid.py b/tardis/montecarlo/montecarlo_numba/nonhomologous_grid.py
new file mode 100644
index 00000000000..e5809f36fc5
--- /dev/null
+++ b/tardis/montecarlo/montecarlo_numba/nonhomologous_grid.py
@@ -0,0 +1,194 @@
+import numpy as np
+from numba import njit
+
+from tardis.montecarlo.montecarlo_numba import (
+    njit_dict_no_parallel,
+)
+
+
+@njit(**njit_dict_no_parallel)
+def velocity(r_packet, numba_model):
+    """
+    Velocity at radius r
+
+    Parameters
+    ----------
+    r_packet: RPacket
+    numba_model: NumbaModel
+
+    Returns
+    -----------
+    v: float, current velocity
+    """
+    shell_id = r_packet.current_shell_id
+    v_inner = numba_model.v_inner[shell_id]
+    v_outer = numba_model.v_outer[shell_id]
+    r_inner = numba_model.r_inner[shell_id]
+    r_outer = numba_model.r_outer[shell_id]
+    r = r_packet.r
+    frac = (v_outer-v_inner)/(r_outer-r_inner)
+    return v_inner + frac * (r - r_inner)
+
+@njit(**njit_dict_no_parallel)
+def dv_dr(r_packet, numba_model):
+    """
+    dv/dr for the current shell
+
+    Parameters
+    ----------
+    r_packet: RPacket
+    numba_model: NumbaModel
+
+    Returns
+    -----------
+    dvdr: float, dv/dr of the current shell
+    """
+    shell_id = r_packet.current_shell_id
+    v_inner = numba_model.v_inner[shell_id]
+    v_outer = numba_model.v_outer[shell_id]
+    r_inner = numba_model.r_inner[shell_id]
+    r_outer = numba_model.r_outer[shell_id]
+    return (v_outer-v_inner)/(r_outer-r_inner)
+
+
+@njit(**njit_dict_no_parallel)
+def tau_sobolev_factor(r_packet, numba_model, numba_plasma):
+    """
+    The angle and velocity dependent Tau Sobolev factor. Is called when ENABLE_NONHOMOLOGOUS_EXPANSION is set to True.
+
+    Parameters
+    ----------
+    r_packet: RPacket
+    numba_model: NumbaModel
+    numba_plasma: NumbaPlasma
+
+    Returns
+    -----------
+    factor = //put the equation here
+    """
+    shell_id = r_packet.current_shell_id
+
+    v = velocity(r_packet, numba_model)
+    r = r_packet.r
+    dvdr = dvdr(r_packet, numba_model)
+    mu = r_packet.mu
+    factor = 1.0/((1 - mu * mu)*v/r + mu * mu * dvdr)
+    return factor
+
+
+@njit(**njit_dict_no_parallel)
+def roots(a, b, c, d, e, threshold):
+    '''
+    Solves ax^4 + bx^3 + cx^2 + dx + e = 0, for the real roots greater than the threshold returns (x - threshold).
+
+    Parameters
+    -----------
+    a, b, c, d, e: coefficients of the equations ax^4 + bx^3 + cx^2 + dx + e = 0, float
+    threshold: lower needed limit on roots, float
+
+    '''
+    delta = 256*a**3*e**3 - 192*a**2*b*d*e**2 - 128*a**2*c**2*e**2 + 144*a**2*c*d**2*e - 27*a**2*d**4 + 144*a*b**2*c*e**2 - 6*a*b**2*d**2*e - 80*a*b*c**2*d*e + 18*a*b*c*d**3 + 16*a*c**4*e - 4*a*c**3*d**2 - 27*b**4*e**2 + 18*b**3*c*d*e - 4*b**3*d**3 - 4*b**2*c**3*e + b**2*c**2*d**2
+    P = 8*a*c - 3*b**2
+    R = b**3 + 8*d*a**2 - 4*a*b*c
+    delta_0 = c**2 - 3*b*d + 12*a*e
+    delta_1 = 2*c**3 - 9*b*c*d + 27*a*d**2 + 27*b**2*e -72*a*c*e
+    D = 64*a**3*e - 16*a**2*c**2 + 16*a*b**2*c - 16*a**2*b*d - 3*b**4
+    p = (8*a*c - 3*b**2)/(8*a**2)
+    q = (b**3 - 4*a*b*c + 8*a**2*d)/(8*a**3)
+    # print("delta: ", delta)
+    # print("P: ", P)
+    # print("R: ", R)
+    # print("delta0: ", delta_0)
+    # print("D: ", D)
+    # print("p: ", p)
+    # print("q: ", q)
+    # print("delta1: ", delta_1)
+    x_1, x_2, x_3, x_4 = None, None, None, None
+    if (delta<0):#do care
+
+        Q = np.cbrt((delta_1 + np.sqrt(delta_1**2 - 4 * delta_0**3))/2)
+        S = np.sqrt(-2/3 * p + 1/(3*a) * (Q + delta_0/Q))/2
+        
+        if -4*S**2 - 2*p - q/S >= 0:
+            x_1 = -b/(4*a) + S + 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            x_2 = -b/(4*a) + S - 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            x_3 = None
+            x_4 = None
+        if -4*S**2 - 2*p + q/S>=0:
+            x_3 = -b/(4*a) - S + 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            x_4 = -b/(4*a) - S - 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            x_1 = None
+            x_2 = None
+        # print("two distinct real roots")
+    elif (delta>0):
+        if (P<0 and D<0): #do care
+            phi = np.arccos(delta_1/(2 * np.sqrt(delta_0**3)))
+            S = np.sqrt(-2/3 * p + 2/(3*a) *np.sqrt(delta_0) * np.cos(phi/3) )/2
+            x_1 = -b/(4*a) + S + 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            x_2 = -b/(4*a) + S - 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            x_3 = -b/(4*a) - S + 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            x_4 = -b/(4*a) - S - 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            # print("four distinct real roots")
+        elif(P>0 and D>0): #do not care
+            
+            print("all complex")
+    else:
+        if (P<0 and D<0 and delta_0!= 0): #do care
+            phi = np.arccos(delta_1/(2 * np.sqrt(delta_0**3)))
+            S = np.sqrt(-2/3 * p + 2/(3*a) *np.sqrt(delta_0) * np.cos(phi/3) )/2
+            x_1 = -b/(4*a) + S + 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            x_2 = -b/(4*a) + S - 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            x_3 = -b/(4*a) - S + 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            x_4 = -b/(4*a) - S - 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            # print("one real double root and two real simple roots")
+        elif(D>0) or (P>0 and (D!=0 or R!=0)): #do care
+            phi = np.arccos(delta_1/(2 * np.sqrt(delta_0**3)))
+            S = np.sqrt(-2/3 * p + 2/(3*a) *np.sqrt(delta_0) * np.cos(phi/3) )/2
+            if -4*S**2 - 2*p - q/S >= 0:
+                x_1 = -b/(4*a) + S + 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+                x_2 = -b/(4*a) + S - 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            if -4*S**2 - 2*p + q/S>=0:
+                x_3 = -b/(4*a) - S + 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+                x_4 = -b/(4*a) - S - 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            # print("one real double root and two complex")
+        elif (delta_0 == 0 and D!=0): #do care
+            # print("one triple real root and one simple real root")
+            phi = np.arccos(delta_1/(2 * np.sqrt(delta_0**3)))
+            S = np.sqrt(-2/3 * p + 2/(3*a) *np.sqrt(delta_0) * np.cos(phi/3) )/2
+            x_1 = -b/(4*a) + S + 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            x_2 = -b/(4*a) + S - 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+            x_3 = -b/(4*a) - S + 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            x_4 = -b/(4*a) - S - 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+        elif (D == 0):
+            if (P < 0):
+                print("two double real roots") #do care
+                phi = np.arccos(delta_1/(2 * np.sqrt(delta_0**3)))
+                S = np.sqrt(-2/3 * p + 2/(3*a) *np.sqrt(delta_0) * np.cos(phi/3) )/2
+                x_1 = -b/(4*a) + S + 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+                x_2 = -b/(4*a) + S - 1/2 * np.sqrt(-4*S**2 - 2*p - q/S)
+                x_3 = -b/(4*a) - S + 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+                x_4 = -b/(4*a) - S - 1/2 * np.sqrt(-4*S**2 - 2*p + q/S)
+            elif (P > 0 and R == 0):
+                print("all complex") #don't care
+            elif (delta_0 == 0):
+                print("all four equal to: ", -b/(4*a)) #do care
+                x_1 = -b/(4*a)
+    roots = []
+    if x_1 != None:
+        x_1 -= threshold
+        if x_1 > 0:
+            roots.append(x_1)
+    if x_2 != None:
+        x_2 -= threshold
+        if x_2 > 0:
+            roots.append(x_2)
+    if x_3 != None:
+        x_3 -= threshold
+        if x_3 > 0:
+            roots.append(x_3)
+    if x_4 != None:
+        x_4 -= threshold
+        if x_4 > 0:
+            roots.append(x_4)
+    
+    return roots
diff --git a/tardis/montecarlo/montecarlo_numba/numba_config.py b/tardis/montecarlo/montecarlo_numba/numba_config.py
index 4bceebf975c..132d206f2d1 100644
--- a/tardis/montecarlo/montecarlo_numba/numba_config.py
+++ b/tardis/montecarlo/montecarlo_numba/numba_config.py
@@ -8,3 +8,4 @@
 H = const.h.cgs.value
 
 ENABLE_FULL_RELATIVITY = False
+ENABLE_NONHOMOLOGOUS_EXPANSION = False