def isotope (self,before="POMASS = 1.000", change="POMASS = 3.000"):
from pymatgen.io.vasp.outputs import Poscar
from pymatgen.io.vasp.inputs import PotcarSingle
from pymatgen.io.vasp.outputs import Potcar
from pymatgen.io.vasp.sets import MPNMRSet
import shutil
import os
os.chdir(self.dire)
print (os.getcwd())
poscar= Poscar.from_file("POSCAR")
# structure = poscar.structure
print (poscar)
potcar= Potcar.from_file("POTCAR")
potc=str(potcar)
cc= potc.replace(before,change)
vv=PotcarSingle(cc,)
vv.write_file("POTCAR_1")
print (vv)
