QChem

Q-Chem is a commercial quantum chemistry package available for use in our group.

Fragment Guesses with multiple bases

To uses basis set projection and fragment guesses you have to run the smaller basis set calculation with fragments first, and then read this into a second calculation which does the projection

 $molecule
    0 1
 --
 0 1
    H       0.000000    -0.115747     1.133769
    H       0.000000     1.109931    -0.113383
    O       0.000000     0.005817    -0.020386
 --
 0 1
    He      10000   0 0
 $end
 $rem
    JOBTYPE              sp
    EXCHANGE             b3lyp
    BASIS                cc-pVDZ
    scf_guess fragmo
    CORRELATION          none
    SYMMETRY             false
    SYM_IGNORE           true
 $end
 @@@
 $molecule
 read
 $end
 $rem
    scf_guess read
    JOBTYPE              sp
    EXCHANGE             b3lyp
    BASIS                cc-pVTZ
    BASIS2                cc-pVDZ
    CORRELATION          none
    MAX_SCF_CYCLES       600
    SYMMETRY             false
    SYM_IGNORE           true
    basisprojtype ovprojection
 $end

INTDUMP / FCIDUMP

To create files in the FCIDUMP format (used e.g. by Hande) create a QChem input file:

$molecule
0 1
  H 0 0 0
  H 1 0 0
$end

$rem
  correlation idump
  exchange hf
  basis sto-3G
  use_abelian_subgroup true
  scf_guess_print 9999
$end

and use the version of QChem currently (2020-02-20) available from:

[you@liminal] $ export QC=public_o
[you@liminal] $ source ~ajwt3/code/qchem/qcsetup.bash
[you@liminal] $ qchem your-input-file.in

This should produce a file called INTDUMP

 &FCI NORB= 2,NELEC=  2,MS2=  0
  ORBSYM=1,6,
  ISYM=1 UHF=.FALSE.
 &END
     0.62640249948715   1   1   1   1
     0.19679058349422   1   2   1   2
      0.6217067630807   2   2   1   1
     0.19679058349422   2   1   1   2
     0.19679058349422   2   1   2   1
     0.65307074688898   2   2   2   2
     -1.1108441795661   1   1   0   0
    -0.58912100326925   2   2   0   0
    -0.39980670964291   1   0   0   0
     0.37823811728909   2   0   0   0
     0.52917721026434   0   0   0   0

Troubleshooting SCF Metadynamics Calculations

If a calculation repeatedly converges on the same solution, adjusting one or more of these keywords can help to find different solutions.

• Increasing SCF_MINFIND_INITNORM, SCF_MINFIND_INITLAMBDA and SCF_MINFIND_INREASEFACTOR can help get out of the minimum. However, setting SCF_MINFIND_INITLAMBDA too high can result in SCF solutions with very high energies, and SCF_MINFIND_INITNORM can get in the way of convergence, particularly when reading in a solution (in that case, set it to zero).
• Increasing SCF_MINFIND_RESTARTSTEPS helps if a solution is close to another minima but does not converge before the calculation restarts with new orbitals
• Changing SCF_MINFIND_RANDOMMIXING is generally helpful. +/- 15708 and 31416 are sensible values to try, and 07854 (which corresponds to halfway between swapping the orbitals) can also help.

If the resulting solutions are too high in energy,

• decreasing SCF_MINFIND_INITNORM
• decreasing SCF_MINFIND_INITLAMBDA
• decreasing SCF_MINFIND_MIXENERGY (even to 00005)
• adjusting MOM_START
• and using SCF_MINFIND_MIXMETHOD 1 or 2

can help to find lower-energy excited states.