Get composition of output structure
        
        Returns:
            String - Composition based on output structure

def get_composition(self):
        '''Get composition of output structure
        
        Returns:
            String - Composition based on output structure
        '''
        strc = self.get_output_structure()
        counts = Counter(strc.get_chemical_symbols())
        return ''.join(k if counts[k]==1 else '%s%d'%(k,counts[k]) \
                for k in sorted(counts))