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Reading a Q-Chem output

Guide 3 · where to find the four numbers you need

A Q-Chem output file is long, but for this lab you need only four numbers from each job. Use your text editor’s search (Ctrl+F / ⌘F) with the phrases below.

You need Search for Example line
Total energy (hartree) Total energy in the final basis set Total energy in the final basis set = -920.6431907520
Number of SCF cycles Convergence criterion met the cycle number at the start of that line, e.g. 12
Wall time (seconds) Total job time Total job time: 14.52s(wall), 52.10s(cpu) → use 14.52
Number of basis functions basis functions There are 26 shells and 74 basis functions → 74

An annotated example

Here is an abridged output for a single-point energy, with the important lines highlighted.

 ...
 -----------------------------------------------------------------------
  General SCF calculation program by
  ...
 -----------------------------------------------------------------------
 There are 26 shells and 74 basis functions          ← number of basis functions
 ...
 ---------------------------------------
  Cycle       Energy         DIIS error
 ---------------------------------------
    1    -920.4153225231      4.31e-02
    2    -920.6017766913      7.45e-03
    ...
   11    -920.6431907518      1.12e-07
   12    -920.6431907520      4.05e-09  Convergence criterion met   ← 12 SCF cycles
 ---------------------------------------
 SCF time:   CPU 41.36s  wall 11.73s
 SCF   energy in the final basis set =     -920.6431907520
 Total energy in the final basis set =     -920.6431907520   ← total energy (Eh)
 ...
 Total job time:  14.52s(wall), 52.10s(cpu)   ← wall time
 ...
        *  Thank you very much for using Q-Chem.  Have a nice day.  *
NoteSCF energy or total energy?

For plain DFT and HF the two lines are usually identical. With extra energy terms, such as a dispersion correction (-D4), they can differ, and the total energy is the one that includes everything. Always use the total energy.

Sanity checks before you trust a number

  • Did the job finish? The last lines should say “Thank you very much for using Q-Chem. Have a nice day.” If they don’t, the job crashed or ran out of time.
  • Did the SCF converge? Look for Convergence criterion met. If you see a warning that the SCF failed to converge, the energy is meaningless.
  • Right charge and multiplicity? Near the top, Q-Chem repeats the $molecule section. A triplet O atom run as a singlet gives a perfectly converged, perfectly wrong energy.
  • Right method and basis? The $rem section is echoed at the top as well. One typo (e.g. def2-QZVP instead of def2-SVP) changes everything.
  • Do the significant figures look sensible? Copy all the digits Q-Chem prints. Reaction energies are small differences between big numbers, so rounding total energies to 4 decimals adds errors of up to 0.03 kcal/mol.
TipWall time vs CPU time

Wall time is what you’d measure with a stopwatch; CPU time adds up the time of every processor core used. On a multi-core server CPU time is larger. In this lab we always compare wall times, and only between jobs run on the same server.

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