There are two methods by which MO coefficients can be used from a previous job by reading them from disk:
Running two independent jobs sequentially invoking Q-Chem with three command line variables:.
localhost-1> qchem job1.in job1.out save localhost-2> qchem job2.in job2.out save
Note: (1) The $rem variable SCF_GUESS must be set to READ in job2.in. (2) Scratch files remain in $QCSCRATCH/save on exit.
Running a batch job where two jobs are placed into a single input file
separated by the string @@@ on a single line.
Note: (1) SCF_GUESS must be set to READ in the second job of the batch file. (2) A third Q-Chem command line variable is not necessary. (3) As for the SAD guess, Q-Chem requires at least two SCF cycles to ensure proper SCF convergence (idempotency of the density).
Note: It is up to the user to make sure that the basis sets match between the two jobs. There is no internal checking for this, although the occupied orbitals are re-orthogonalized in the current basis after being read in. If you want to project from a smaller basis into a larger basis, consult Section 4.4.5.
$comment OH radical, part 1. Do initial iterations of cation orbitals. $end $molecule 1 1 O 0.000 0.000 0.000 H 0.000 0.000 1.000 $end $rem BASIS = 6-311++G(2df) METHOD = hf THRESH = 10 MAX_SCF_CYCLES = 2 SCF_CONVERGENCE = 1 $end @@@ $comment OH radical, part 2. Read cation orbitals, do the radical $end $molecule 0 2 O 0.000 0.000 0.000 H 0.000 0.000 1.000 $end $rem BASIS = 6-311++G(2df) METHOD = hf UNRESTRICTED = false SCF_ALGORITHM = dm SCF_CONVERGENCE = 7 SCF_GUESS = read THRESH = 10 $end