4.4 SCF Initial Guess

4.4.3 Reading MOs from Disk

There are two methods by which MO coefficients can be used from a previous job by reading them from disk:

  1. 1.

    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.

  2. 2.

    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.

Example 4.5  Input for an ROHF calculation on the OH radical. One SCF cycle is initially performed on the cation, to get reasonably good initial guess orbitals, which are then read in as the guess for the radical. This avoids the use of Q-Chem’s default GWH guess for ROHF, which is often poor.

$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