def METHOD_NAME(compDict, percentCutoff):
  """
    Filters isotopes with less than percentCutoff for easier calculation.
    @ In, compDict, dictionary, key=isotope
                                value=atomic density
    @ In, percentCutoff, float, cutoff threshold for ignoring isotopes (0 -1)
    @ Out, comptDict, dictionary, key=isotope
                                  value=compDict
  """
  # check if percentCutoff value is valid
  if percentCutoff < 0 or percentCutoff > 1:
    raise ValueError('Percent has to be between 0 and 1')
  # calculate atomicDensityCutoff
  totalAtomicDensity = sum(compDict.values())
  atomicDensityCutoff = percentCutoff * totalAtomicDensity
  # delete list since cannot change dictionary during iteration
  deleteList = []
  for key, atomDensity in compDict.items():
    if atomDensity < atomicDensityCutoff:
      deleteList.append(key)
  # delete the isotopes with less than percentCutoff
  for isotope in deleteList:
    del compDict[isotope]
  return compDict