# get formula dictionary
dd = {}
for ca in self.chemAtoms:
if isinstance(ca, ChemAtom):
ss = ca.elementSymbol
ii = dd.get(ss)
if ii is None:
dd[ss] = 1
else:
dd[ss] = ii + 1
# calculate mass
xx = self.root.currentChemElementStore
result = sum(ii * xx.findFirstChemElement(symbol=ss).mass for (ss, ii) in dd.iteritems())