from ase.build import molecule
from ase.neb import NEB

initial = molecule('C2H6')
final = initial.copy()
final.positions[2:5] = initial.positions[[3, 4, 2]]

images = [initial]
for i in range(5):
    images.append(initial.copy())
images.append(final)

neb = NEB(images)
d0 = images[3].get_distance(2, 3)
neb.interpolate()
d1 = images[3].get_distance(2, 3)
neb.idpp_interpolate(fmax=0.005)
d2 = images[3].get_distance(2, 3)
print(d0, d1, d2)
assert abs(d2 - 1.74) < 0.01