Set the box size for the molecular assembly

def _set_box(self):
        """
        Set the box size for the molecular assembly
        """
        net_volume = 0.0
        for idx, mol in enumerate(self.mols):
            length = max([np.max(mol.cart_coords[:, i])-np.min(mol.cart_coords[:, i])
                           for i in range(3)]) + 2.0
            net_volume += (length**3.0) * float(self.param_list[idx]['number'])
        length = net_volume**(1.0/3.0)
        for idx, mol in enumerate(self.mols):
            self.param_list[idx]['inside box'] = '0.0 0.0 0.0 {} {} {}'.format(
                length, length, length)