# 4.3.2 Additional Options

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 ${\cal{O}}({N})$ 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 $\leq 14$).
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 $\leq 14$).
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).