Q-Chem 5.0 User’s Manual

3.6 Multiple Jobs in a Single File: Q-Chem Batch Jobs

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 ($i$) use information obtained in a prior job (i.e., an optimized geometry) in a subsequent job; or ($ii$) keep related calculations together in a single output file.

Some limitations should be kept in mind:

The following example requests a batch job consisting of ($i$) a HF/6-31G* geometry optimization; followed by ($ii$) 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 ($iii$) a single-point calculation at the same geometry but at a higher level of theory, MP2/6-311G(d,p).

Example 3.16  Example of using information from previous jobs in a single input file.

$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