def filter_out_hydrogens(mol):
    """Get indices for non-hydrogen atoms.

    Parameters
    ----------
    mol : rdkit.Chem.rdchem.Mol
        RDKit molecule instance.

    Returns
    -------
    indices_left : list of int
        Indices of non-hydrogen atoms.
    """
    indices_left = []
    for i, atom in enumerate(mol.GetAtoms()):
        atomic_num = atom.GetAtomicNum()
        if atomic_num != 1:
            indices_left.append(i)
    return indices_left
