Hello,

I am new to fenics and am trying to do some 3d nonlinear finite element analysis on an equation for cancer modeling. I am running into an error that I can’t seem to find a fix for on the internet. I was hoping someone can help - it’s probably something easy that I just don’t understand. If a fix is found I would love some explanation on why it solves the problem (and what this error even means ).

Here is the error verbatim:

*** Error: Unable to successfully call PETSc function ‘MatSetValuesLocal’.

*** Reason: PETSc error code is: 63 (Argument out of range).

*** Where: This error was encountered inside /tmp/dolfin/dolfin/la/PETScMatrix.cpp.

*** Process: 0

And here is my code:

```
from __future__ import print_function
from fenics import *
import matplotlib.pyplot as plt
import time
T = 2.0 # final time
num_steps = 50 # number of time steps
dt = T / num_steps # time step size
# Create mesh and define function space
# mesh = RectangleMesh(Point(-2, -2), Point(2, 2), nx, ny)
mesh = UnitCubeMesh(16, 16, 16)
V = FunctionSpace(mesh, 'P', 1)
# Define boundary condition
# def boundary(x, on_boundary):
# return on_boundary
#
# bc = DirichletBC(V, Constant(0), boundary)
# Define initial value
u_0 = Expression('exp(1 / (1 - pow(pow(x[0], 2) + pow(x[1], 2) + pow(x[2], 2), 2) ) )', degree=2)
u_n = interpolate(u_0, V)
# Define variational problem
u = Function(V)
v = TestFunction(V)
D = Constant((1/1000)/.18)
a = Constant(1/.18)
beta = Constant(0.9)
one = Constant(1)
F = u*v*dx + dt*D*dot(grad(u), grad(v))*dx - (u_n + dt*a*(one - beta*u)*u)*v*dx
# Create VTK file for saving solution
vtkfile = File('CarTCellModel/solution.pvd')
# Time-stepping
u = Function(V)
t = 0
for n in range(num_steps):
# Update current time
t += dt
# Compute solution
solve(F == 0, u)
# Save to file and plot solution
vtkfile << (u, t)
plot(u)
# Update previous solution
u_n.assign(u)
# Hold plot
#interactive()
# hold plot
#plt.show()
```

Let me know if you need any more info. If it helps, most of this code was adapted from the Gaussian Heat equation example here: fenics-tutorial/ft04_heat_gaussian.py at master · hplgit/fenics-tutorial · GitHub. I then tried to modify it to work on 3D and for the nonlinear weak form.