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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
${10}^{-\mathrm{SCF}\mathrm{\_}\mathrm{CONVERGENCE}}$. 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.

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.