Hi dokken, thanks for your quick reply and sorry for not stating the issue properly. Here’s the minimal work example, basically copied from demo-chan-hillilard.py from https://fenicsproject.org/olddocs/dolfin/2019.1.0/python/demos/cahn-hilliard/demo_cahn-hilliard.py.html

```
import random
from dolfin import *
# Class representing the intial conditions
class InitialConditions(UserExpression):
def __init__(self, **kwargs):
random.seed(2 + MPI.rank(MPI.comm_world))
super().__init__(**kwargs)
def eval(self, values, x):
values[0] = 0.63 + 0.02*(0.5 - random.random())
values[1] = 0.0
def value_shape(self):
return (2,)
# Class for interfacing with the Newton solver
class CahnHilliardEquation(NonlinearProblem):
def __init__(self, a, L):
NonlinearProblem.__init__(self)
self.L = L
self.a = a
def F(self, b, x):
assemble(self.L, tensor=b)
def J(self, A, x):
assemble(self.a, tensor=A)
# Model parameters
lmbda = 1.0e-02 # surface parameter
dt = 5.0e-06 # time step
theta = 0.5 # time stepping family, e.g. theta=1 -> backward Euler, theta=0.5 -> Crank-Nicolson
# Form compiler options
parameters["form_compiler"]["optimize"] = True
parameters["form_compiler"]["cpp_optimize"] = True
# Create mesh and build function space
mesh = UnitSquareMesh.create(200, 200, CellType.Type.quadrilateral)
P1 = FiniteElement("Lagrange", mesh.ufl_cell(), 1)
ME = FunctionSpace(mesh, P1*P1)
# Define trial and test functions
du = TrialFunction(ME)
q, v = TestFunctions(ME)
# Define functions
u = Function(ME) # current solution
u0 = Function(ME) # solution from previous converged step
# Split mixed functions
dc, dmu = split(du)
c, mu = split(u)
c0, mu0 = split(u0)
# Create intial conditions and interpolate
u_init = InitialConditions(degree=1)
u.interpolate(u_init)
u0.interpolate(u_init)
# Compute the chemical potential df/dc
c = variable(c)
f = 100*c**2*(1-c)**2
dfdc = diff(f, c)
# mu_(n+theta)
mu_mid = (1.0-theta)*mu0 + theta*mu
# which is then used in the definition of the variational forms::
# Weak statement of the equations
L0 = c*q*dx - c0*q*dx + dt*dot(grad(mu_mid), grad(q))*dx
L1 = mu*v*dx - dfdc*v*dx - lmbda*dot(grad(c), grad(v))*dx
L = L0 + L1
# Compute directional derivative about u in the direction of du (Jacobian)
a = derivative(L, u, du)
# Create nonlinear problem and Newton solver
problem = CahnHilliardEquation(a, L)
comm = MPI.comm_world
rank = MPI.rank(comm)
import numpy as np
class CustomSolver(NewtonSolver):
def __init__(self):
NewtonSolver.__init__(self)
def _clip(self, x):
MPI.barrier(comm)
if rank == 0:
_size = int(round(x.size()/2))
print(_size)
_c = np.reshape(x[0::2], (_size, 1)) # even index in x is c
_mu = np.reshape(x[1::2], (_size, 1)) # od index in x is mu
_tol = 1e-6
_c = _c.clip(max=1.0-_tol, min=_tol)
_x_clipped = np.concatenate((_c, _mu), axis=1).flatten()
x = Vector(MPI.comm_self,_c.size*2)
x.set_local(_x_clipped)
return x
def update_solution(self, x, dx, relaxation_parameter, nonlinear_problem,
iteration):
x.axpy(-0.01, dx)
x = self._clip(x)
solver = CustomSolver()
solver.parameters["linear_solver"] = "lu"
solver.parameters["convergence_criterion"] = "incremental"
solver.parameters["relative_tolerance"] = 1e-6
# Output file
file = File("output.pvd", "compressed")
# Step in time
t = 0.0
T = 50*dt
while (t < T):
t += dt
u0.vector()[:] = u.vector()
solver.solve(problem, u.vector())
file << (u.split()[0], t)
```

when I type `python3 demo_cahn-hilliard.py`

everything works fine, but when I type `mpirun -np 4 demo_cahn-hilliard.py`

the following error pops out:

`Traceback (most recent call last): File "/Users/archieyao/Desktop/test/demo_cahn-hilliard.py", line 128, in <module> solver.solve(problem, u.vector()) File "/Users/archieyao/Desktop/test/demo_cahn-hilliard.py", line 112, in update_solution x = self._clip(x) ^^^^^^^^^^^^^ File "/Users/archieyao/Desktop/test/demo_cahn-hilliard.py", line 101, in _clip _c = np.reshape(x[0::2], (_size, 1)) # even index in x is c ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/Users/archieyao/tools/miniconda3/envs/ocv_pf/lib/python3.12/site-packages/numpy/core/fromnumeric.py", line 285, in reshape return _wrapfunc(a, 'reshape', newshape, order=order) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/Users/archieyao/tools/miniconda3/envs/ocv_pf/lib/python3.12/site-packages/numpy/core/fromnumeric.py", line 59, in _wrapfunc return bound(*args, **kwds) ^^^^^^^^^^^^^^^^^^^^ ValueError: cannot reshape array of size 10089 into shape (40401,1) `