4.3.3 Additional Options

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:
       The value should satisfy THRESH $\ge$ 3 + SCF_CONVERGENCE, although THRESH = 4 + SCF_CONVERGENCE is the default (in most cases) since Q-Chem v. 6.0. See Ref.  459 Gray M., Bowling P. E., Herbert J. M.
J. Phys. Chem. A
(2024), 128, pp. 7739. Link for recommended values of THRESH in the presence of diffuse basis functions, where tighter thresholds are often required and too-loose thresholds may lead to slower convergence or convergence failure.

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 , orbital kinetic energies, and DFT exchange-correlation matrices) on each cycle.
RECOMMENDATION:
       Proceed with care; can result in extremely large output files at level 2 or higher. Output of all information is only available in scfman (GEN_SCFMAN = FALSE). If GEN_SCFMAN is set to TRUE and SCF_PRINT > 1, only level 1 plus MO matrices are available in the output. 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 matrix.
RECOMMENDATION:
       The break-down of energies is often useful (level 1).