Listed below are a number of useful options to customize an SCF calculation. This is only a short summary of the function of these $rem variables. A full list of all SCF-related variables is provided in Appendix C. Several important sub-topics are discussed separately, including methods for large molecules (Section 4.6), customizing the initial guess (Section 4.4), and converging the SCF calculation (Section 4.5).
INTEGRALS_BUFFER
Controls the size of in-core integral storage buffer.
TYPE:
INTEGER
DEFAULT:
15
15 Megabytes.
OPTIONS:
User defined size.
RECOMMENDATION:
Use the default, or consult your systems administrator for hardware limits.
DIRECT_SCF
Controls direct SCF.
TYPE:
LOGICAL
DEFAULT:
Determined by program.
OPTIONS:
TRUE
Forces direct SCF.
FALSE
Do not use direct SCF.
RECOMMENDATION:
Use the default; direct SCF switches off in-core integrals.
METECO
Sets the threshold criteria for discarding shell-pairs.
TYPE:
INTEGER
DEFAULT:
2
Discard shell-pairs below .
OPTIONS:
1
Discard shell-pairs four orders of magnitude below machine precision.
2
Discard shell-pairs below 10.
RECOMMENDATION:
Use the default.
S2THRESH
Cutoff for neglect of overlap integrals, defined via a two-electron shell-pair threshold of (S2THRESH
).
TYPE:
INTEGER
DEFAULT:
Same as THRESH.
OPTIONS:
for a threshold of .
RECOMMENDATION:
Increase the value of S2THRESH if the program finds negative eigenvalues for the overlap matrix.
THRESH
Cutoff for neglect of two electron integrals. (THRESH
).
TYPE:
INTEGER
DEFAULT:
8
For single point energies.
10
For optimizations and frequency calculations.
14
For coupled-cluster calculations.
OPTIONS:
for a threshold of .
RECOMMENDATION:
Should be at least three greater than SCF_CONVERGENCE. Increase for
more significant figures, at greater computational cost.
STABILITY_ANALYSIS
Performs stability analysis for a HF or DFT solution.
TYPE:
LOGICAL
DEFAULT:
FALSE
OPTIONS:
TRUE
Perform stability analysis.
FALSE
Do not perform stability analysis.
RECOMMENDATION:
Set to TRUE when a HF or DFT solution is suspected to be unstable.
SCF_PRINT
Controls level of output from SCF procedure to Q-Chem output file.
TYPE:
INTEGER
DEFAULT:
0
Minimal, concise, useful and necessary output.
OPTIONS:
0
Minimal, concise, useful and necessary output.
1
Level 0 plus component breakdown of SCF electronic energy.
2
Level 1 plus density, Fock and MO matrices on each cycle.
3
Level 2 plus two-electron Fock matrix components (Coulomb, HF exchange
and DFT exchange-correlation matrices) on each cycle.
RECOMMENDATION:
Proceed with care; can result in extremely large output files at level 2 or higher.
These levels are primarily for program debugging.
SCF_FINAL_PRINT
Controls level of output from SCF procedure to Q-Chem output file at the
end of the SCF.
TYPE:
INTEGER
DEFAULT:
0
No extra print out.
OPTIONS:
0
No extra print out.
1
Orbital energies and break-down of SCF energy.
2
Level 1 plus MOs and density matrices.
3
Level 2 plus Fock and density matrices.
RECOMMENDATION:
The break-down of energies is often useful (level 1).