It is sometimes useful to place a sequence of jobs into a single Q-Chem input file, where the individual inputs should be separated from one another by a line consisting of the string @@@. The output from these jobs is then appended sequentially to a single output file. This is useful to (a) use information obtained in a prior job (i.e., an optimized geometry) in a subsequent job; or (b) keep related calculations together in a single output file.
Some limitations should be kept in mind:
The first job will overwrite any existing output file of the same name in the working directory. Restarting the job will also overwrite any existing file.
Q-Chem reads all the jobs from the input file immediately and stores them. Therefore no changes can be made to the details of subsequent jobs following command-line initiation of Q-Chem, even if these subsequent jobs have not yet run.
If any single job fails, Q-Chem proceeds to the next job in the batch file, for good or ill.
No check is made to ensure that dependencies are satisfied, or that information is consistent. For example, in a geometry optimization followed by a frequency calculation, no attempt is made by the latter to check that the optimization was successful. When reading MO coefficients from a previous job, it is the user’s responsibility to ensure that the basis set is the same in both calculations, as this is assumed by the program.
Scratch files are saved from one job to the next in a batch job, so that information from previous jobs can be shared with subsequent ones, but are deleted upon completion of the entire batch job unless the –save command-line argument is supplied, as discussed in Chapter 2.
The following example requests a batch job consisting of () a HF/6-31G* geometry optimization; followed by () a frequency calculation at the same level of theory that uses the previously-optimized geometry (and also reads in the final MOs from the optimization job); and finally () a single-point calculation at the same geometry but at a higher level of theory, MP2/6-311G(d,p).
$comment Optimize H-H at HF/6-31G* $end $molecule 0 1 H H 1 r r = 1.1 $end $rem JOBTYPE opt Optimize the bond length METHOD hf BASIS 6-31G* $end @@@ $comment Now calculate the frequency of H-H at the same level of theory. $end $molecule read $end $rem JOBTYPE freq Calculate vibrational frequency METHOD hf BASIS 6-31G* SCF_GUESS read Read the MOs from disk $end @@@ $comment Now a single point calculation at at MP2/6-311G(d,p)//HF/6-31G* $end $molecule read $end $rem METHOD mp2 BASIS 6-311G(d,p) $end