4.5.2 Basic Convergence Control Options

MAX_SCF_CYCLES
       Controls the maximum number of SCF iterations permitted.
TYPE:
       INTEGER
DEFAULT:
       50
OPTIONS:
       $n$ $n>0$ User-selected.
RECOMMENDATION:
       Increase for slowly converging systems such as those containing transition metals.

SCF_ALGORITHM
       Algorithm used for converging the SCF.
TYPE:
       STRING
DEFAULT:
       DIIS Pulay DIIS.
OPTIONS:
       DIIS Pulay DIIS. DM Direct minimizer. DIIS_DM Uses DIIS initially, switching to direct minimizer for later iterations (See THRESH_DIIS_SWITCH, MAX_DIIS_CYCLES). DIIS_GDM Use DIIS and then later switch to geometric direct minimization (See THRESH_DIIS_SWITCH, MAX_DIIS_CYCLES). LS_DIIS Uses level-shifting initially, switching to DIIS for later iterations (See THRESH_LS_SWITCH, MAX_LS_CYCLES). GDM Geometric Direct Minimization. RCA Relaxed constraint algorithm RCA_DIIS Use RCA initially, switching to DIIS for later iterations (see THRESH_RCA_SWITCH and MAX_RCA_CYCLES). ROOTHAAN Roothaan repeated diagonalization.
RECOMMENDATION:
       Use DIIS unless performing a restricted open-shell calculation, in which case GDM is recommended. If DIIS fails to find a reasonable approximate solution in the initial iterations, RCA_DIIS is the recommended fallback option. If DIIS approaches the correct solution but fails to finally converge, DIIS_GDM is the recommended fallback. For systems with small HOMO-LUMO gaps and DIIS fails to converge, LS_DIIS could help.

SCF_CONVERGENCE
       SCF is considered converged when the wave function error is less that ${10}^{-\mathrm{SCF}\mathrm{\_}\mathrm{CONVERGENCE}}$. Adjust the value of THRESH at the same time. Note as of Q-Chem 3.0 the DIIS error is measured by the maximum error rather than the RMS error.
TYPE:
       INTEGER
DEFAULT:
       5 For single point energy calculations. 7 For geometry optimizations and vibrational analysis. 8 For SSG calculations, see Chapter 6.
OPTIONS:
       $n$ Corresponding to ${10}^{-n}$
RECOMMENDATION:
       Tighter criteria for geometry optimization and vibration analysis. Larger values provide more significant figures, at greater computational cost.