It is often the case that a single algorithm is not able to guarantee SCF convergence. Meanwhile, some SCF algorithms (e.g., ADIIS) can accelerate convergence at the beginning of an SCF calculation but becomes less efficient near the convergence. While a few hybrid algorithms (DIIS_GDM, RCA_DIIS) have been enabled in Q-Chem’s original SCF implementation, in GEN_SCFMAN, we seek for a more flexible setup for the use of multiple SCF algorithms so that users can have a more precise control on the SCF procedure. With the current implementation, at most four distinct algorithms (usually more than enough) can be employed in one single SCF calculation based on GEN_SCFMAN, and the basic job control is as follows:
GEN_SCFMAN_HYBRID_ALGO
GEN_SCFMAN_HYBRID_ALGO
Use multiple algorithms in an SCF calculation based on GEN_SCFMAN.
TYPE:
BOOLEAN
DEFAULT:
FALSE
OPTIONS:
FALSE
Use a single SCF algorithm (given by SCF_ALGORITHM).
TRUE
Use multiple SCF algorithms (to be specified).
RECOMMENDATION:
Set it to TRUE when the use of more than one algorithm is desired.
GEN_SCFMAN_ALGO_1
GEN_SCFMAN_ALGO_1
The first algorithm to be used in a hybrid-algorithm calculation.
TYPE:
STRING
DEFAULT:
0
OPTIONS:
All the available SCF_ALGORITHM options, including the GEN_SCFMAN
additions (Section 4.3).
RECOMMENDATION:
None
GEN_SCFMAN_ITER_1
GEN_SCFMAN_ITER_1
Maximum number of iterations given to the first algorithm. If used up, switch
to the next algorithm.
TYPE:
INTEGER
DEFAULT:
50
OPTIONS:
User-defined
RECOMMENDATION:
None
GEN_SCFMAN_CONV_1
GEN_SCFMAN_CONV_1
The convergence criterion given to the first algorithm. If reached, switch to
the next algorithm.
TYPE:
INTEGER
DEFAULT:
0
OPTIONS:
10
RECOMMENDATION:
None
Note: $rem variables GEN_SCFMAN_ALGO_X, GEN_SCFMAN_ITER_X, GEN_SCFMAN_CONV_X (X = 2, 3, 4) are defined and used in a similar way.
Example 4.15 B3LYP/3-21G calculation for a cadmium-imidazole complex using the ADIIS + DIIS algorithm (an example from Ref. 491). Due to the poor quality of the CORE guess, using a single algorithm such as DIIS or GDM fails to converge.
$molecule 2 1 Cd 0.000000 0.000000 0.000000 N 0.000000 0.000000 -2.260001 N -0.685444 0.000000 -4.348035 C 0.676053 0.000000 -4.385069 C 1.085240 0.000000 -3.091231 C -1.044752 0.000000 -3.060220 H 1.231530 0.000000 -5.300759 H 2.088641 0.000000 -2.711077 H -2.068750 0.000000 -2.726515 H -1.313170 0.000000 -5.174718 $end $rem EXCHANGE B3LYP BASIS 3-21g UNRESTRICTED FALSE SYMMETRY FALSE SYM_IGNORE TRUE THRESH 14 SCF_GUESS CORE GEN_SCFMAN_HYBRID_ALGO TRUE GEN_SCFMAN_ALGO_1 ADIIS GEN_SCFMAN_CONV_1 3 !switch to DIIS when error < 1E-3 GEN_SCFMAN_ITER_1 50 GEN_SCFMAN_ALGO_2 DIIS GEN_SCFMAN_CONV_2 8 GEN_SCFMAN_ITER_2 50 $end