A CloudComPy script to apply ICP registration:

#!/usr/bin/env python3

import os
import sys
import math
import psutil

os.environ["_CCTRACE_"]="ON" # only if you want C++ debug traces

import cloudComPy as cc

objs = cc.importFile("chimeres.bin")
chimere_1_mesh      = objs[0][0] # mesh, first part of the object
chimere_2_mesh      = objs[0][1] # mesh, second part of the object
chimere_1_cloud     = objs[1][0] # cloud, first part of the object
chimere_2_cloud     = objs[1][1] # cloud, second part of the object
chimere_2_manualRot = objs[1][2] # cloud, second part of the object, very approximately aligned (hand rotation with the CloudCompare GUI)

chimere_1_mesh.setName("chimere_1_mesh")
chimere_2_mesh.setName("chimere_2_mesh")
chimere_1_cloud.setName("chimere_1_cloud")
chimere_2_cloud.setName("chimere_2_cloud")
chimere_2_manualRot.setName("chimere_2_manualRot")

# ICP with chimere_1_mesh and chimere_2_cloud does not work (neither with GUI nor with Python): ICP requires some first alignment

# ICP with the second part "pre aligned" by hand: OK, same result as CloudCompare GUI

res=cc.ICP(data=chimere_2_manualRot, model=chimere_1_mesh, finalOverlapRatio=0.5, method=cc.CONVERGENCE_TYPE.MAX_ERROR_CONVERGENCE)

tr1 = res.transMat
print(tr1.toString())
chimere_2_manualRot_transformed = chimere_2_manualRot.cloneThis() # = res.aligned.cloneThis() # note: res.aligned is data!
chimere_2_manualRot_transformed.applyRigidTransformation(tr1)
chimere_2_manualRot_transformed.setName("chimere2_manualRot_transformed_afterICP")

cc.SaveEntities([chimere_1_mesh, chimere_2_manualRot, chimere_2_manualRot_transformed],"chimeres_ICP.bin")