4.5 Converging SCF Calculations

4.5.2 Basic Convergence Control Options

(May 16, 2021)

See also more detailed options in the following sections, and note that the SCF convergence criterion and the integral threshold must be set in a compatible manner, (this usually means THRESH should be set to at least 3 higher than SCF_CONVERGENCE).

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-SCF_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.

In some cases besides the total SCF energy, one needs its separate energy components, like kinetic energy, exchange energy, correlation energy, etc. The values of these components are printed at each SCF cycle if one specifies SCF_PRINT = 1 in the input.