def validate_structure(value, _):
    """
    Validate structure input port. It takes care to check that no alchemical atoms
    are passed, since they are not supported.
    """
    # Even though structure is mandatory here, other workchain might change this requirement
    # (see iterator) and therefore the "if" is still needed.
    if value:
        for kind in value.kinds:
            if len(kind.symbols) > 1:
                return "alchemical atoms not supported; structure must have one single symbol per kind."