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# 4.5.2 Basic Convergence Control Options

(February 4, 2022)

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

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

SCF_ALGORITHM
Algorithm used for converging the SCF.
TYPE:
STRING
DEFAULT:
DIIS Pulay DIIS.
OPTIONS:
DIIS Pulay DIIS. ROOTHAAN Roothaan repeated diagonalization DM Direct minimizer. L_BFGS Limited memory BFGS algorithm with line search GDM Geometric Direct Minimization. GDM_LS L-BFGS algorithm with a GDM-like preconditioner and line search (see also GDM_QLS, which uses a quadratic line search algorithm) RCA Relaxed constraint algorithm ADIIS The combination of the ARH energy function and DIIS by Hu and Yang NEWTON_CG Solve $\mathbf{Hd}=-\mathbf{g}$ using conjugated gradients to determine Newton steps (see also Newton_MINRES, which uses the MINRES algorithm to solve the linear equation) 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) RCA_DIIS Use RCA initially, switching to DIIS for later iterations (See THRESH_RCA_SWITCH and MAX_RCA_CYCLES) ADIIS_DIIS Use ADIIS initially, switching to DIIS for later iterations (See THRESH_ADIIS_SWITCH and MAX_ADIIS_CYCLES)
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 and ADIIS_DIIS are the recommended fallback options. 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.

Note: for the usage of Square Gradient Minimization (SGM) and related algorithm options, see the documentation in Section 4.5.13.

SCF_CONVERGENCE

SCF_CONVERGENCE
SCF is considered converged when the wave function error is less that $10^{-\mathrm{SCF\_CONVERGENCE}}$. 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 (including BSSE and XSAPT jobs) 7 For job types NMR, STATPOLAR, DYNPOLAR, HYPERPOLAR, and ISSC 8 For most other job types, including geometry optimization, transition-state search, vibrational analysis, CIS/TDDFT calculations, correlated wavefunction methods, energy decomposition analysis (EDA2), etc.
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.