4.3 Basic SCF Job Control

4.3.1 Additional Options

(May 16, 2021)

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 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-THRESH.
OPTIONS:
       1 Discard shell-pairs four orders of magnitude below machine precision. 2 Discard shell-pairs below 10-THRESH.
RECOMMENDATION:
       Use the default.

S2THRESH
       Cutoff for neglect of overlap integrals, defined via a two-electron shell-pair threshold of 10-S2THRESH (S2THRESH 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-THRESH (THRESH 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. 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).