fix the lattice issue

Author: qzhu2017

pyxtal/__init__.py

@@ -2229,7 +2229,10 @@ def make_transitions(self, disps, lattice=None, translation=None, N_images=3, bo
                 struc.atom_sites[j].update(coord)
             struc.source = 'Transition {:d} {:6.3f}'.format(i, max_disp*i)
             struc.disp = max_disp*i
-            struc.lattice.set_matrix(cell + i*l_disps)
+            if i >= N_images:
+                struc.lattice.set_matrix(cell + (i-2*(N_images-1))*l_disps)
+            else:
+                struc.lattice.set_matrix(cell + i*l_disps)
             strucs.append(struc)
         return strucs