Error handling

In the previous sections it is always assumed that Q-Chem calculations always finish successfully, but, in general, this is not true. Calculation may fail for several reasons, incorrect input parameters, slow convergence, problems with the cluster, issues during the parser process, etc.. To handle these errors PyQchem implements two custom error classes: OutputError and ParserError.

These errors are risen during the get_output_from_qchem function execution if something has gone wrong. If OutputError is risen means that Q-Chem calculation did not finish correctly. This may mean that the input is incorrect or something happened with the computer that lead the calculation to crash. When this error is risen the traceback will show the last 20 lines of the Q-Chem output where, hopefully, you will find enough information to determine the cause of the error.

Traceback (most recent call last):
  File "/Users/abel/PycharmProjects/qchem_scripts/scripts/test_tddft.py", line 51, in <module>
    store_full_output=True,
  File "/Users/abel/PycharmProjects/qchem_scripts/pyqchem/qchem_core.py", line 330, in get_output_from_qchem
    raise OutputError(output, err)
pyqchem.errors.OutputError: Error in Q-Chem calculation:
sh: gdb: command not found
Unable to call dbx in QTraceback: No such file or directory
 http://arma.sourceforge.net/

 Q-Chem begins on Fri Jun  5 02:31:23 2020

Host:
0

     Scratch files written to /Users/abel/Scratch/export/qchem66428//

 Name = B3LP

 Q-Chem fatal error occurred in module libdft/dftcodes.C, line 599:

 Unrecognized exchange functional in XCode


 Please submit a crash report at q-chem.com/reporter

In some cases, specially when working with complex workflows it becomes useful to be able to capture these errors to handle them automatically without finish all the workflow. This can be done by the usual try/except statements. By capturing the error it is possible to skip the calculation, recover the error_lines and even recover the full Q-Chem output:

try:
    parsed_data = get_output_from_qchem(qc_input,
                                        processors=4,
                                        parser=basic_optimization)
except OutputError as e:
    print('These are the error lines:\n', e.error_lines)
    print('This is the full output:\n', e.full_output)

Using the full output it is possible to try to parse the information that contains by applying the parser directly:

try:
    parsed_data = get_output_from_qchem(qc_input,
                                        processors=4,
                                        parser=basic_optimization)
except OutputError as e:
    print('Something wrong happened!:\n', e.error_lines)
    print('Recovering usable data...')
    parsed_data = basic_optimization(e.full_output)

But if the calculation is really incomplete, or the format of Q-Chem output is incompatible with the parser, the parsing process may also fail and a ParserError will be risen. In this case the output data cannot be recovered using this parser.

In the same way as OutputError a try/except block can be written to capture this error. A sensible to proceed can be either skip the calculation or try another parser by nesting two try/except blocks :

try:
    parsed_data = get_output_from_qchem(qc_input,
                                        processors=4,
                                        parser=basic_optimization)
except OutputError as e:
    print('Something wrong happened!:\n', e.error_lines)
    print('Recovering usable data...')

    try:
        parsed_data = basic_optimization(e.full_output)
    except ParserError:
        print('Trying another parser')
        parsed_data = other_parser(e.full_output)