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 $\mathcal{O}(N)$ 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 ${10}^{-\mathrm{THRESH}}$.

OPTIONS:

1
Discard shell-pairs four orders of magnitude below machine precision.
2
Discard shell-pairs below 10${}^{-\mathrm{THRESH}}$.

RECOMMENDATION:

Use the default.

S2THRESH

Cutoff for neglect of overlap integrals, defined via a two-electron shell-pair threshold of ${10}^{-\mathrm{S2THRESH}}$ (S2THRESH
$\le 14$).

TYPE:

INTEGER

DEFAULT:

Same as THRESH.

OPTIONS:

$n$
for a threshold of ${10}^{-n}$.

RECOMMENDATION:

Increase the value of S2THRESH if the program finds negative eigenvalues for the overlap matrix.

THRESH

Cutoff for neglect of two electron integrals. ${10}^{-\mathrm{THRESH}}$ (THRESH
$\le 14$).

TYPE:

INTEGER

DEFAULT:

8
For single point energies.
10
For optimizations and frequency calculations.
14
For coupled-cluster calculations.

OPTIONS:

$n$
for a threshold of ${10}^{-n}$.

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).