In general, the DIIS procedure (Section 4.5.3) is remarkably successful. One difficulty that is
occasionally encountered is the problem of an SCF that occupies two different
sets of orbitals on alternating iterations, and therefore oscillates and fails
to converge. This can be overcome by choosing orbital occupancies that
maximize the overlap of the new occupied orbitals with the set previously
occupied. Q-Chem contains the maximum overlap method
(MOM),^{Gilbert:2008} developed by Andrew Gilbert and Peter Gill. With
GEN_SCFMAN, the MOM algorithm can be applied to R, U, and RO SCF calculations when
paired with the DIIS algorithm.

MOM is therefore is a useful adjunct to DIIS in convergence problems involving
flipping of orbital occupancies. It is controlled by the *$rem* variable
MOM_START, which specifies the SCF iteration on which the MOM
procedure is first enabled. There are two strategies that are useful in setting
a value for MOM_START. To help maintain an initial configuration it
should be set to start on the first cycle. On the other hand, to assist
convergence it should come on later to avoid holding on to an initial
configuration that may be far from the converged one.

The MOM-related *$rem* variables in full are the following:

MOM_PRINT

Switches printing on within the MOM procedure.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

FALSE
Printing is turned off
TRUE
Printing is turned on.

RECOMMENDATION:

None

MOM_START

Determines when MOM is switched on to stabilize DIIS iterations.

TYPE:

INTEGER

DEFAULT:

0 (FALSE)

OPTIONS:

0 (FALSE)
MOM is not used
$n$
MOM begins on cycle $n$.

RECOMMENDATION:

Set to 1 if preservation of initial orbitals is desired. If MOM is to be
used to aid convergence, an SCF without MOM should be run to determine when
the SCF starts oscillating. MOM should be set to start just before the
oscillations.

MOM_METHOD

Determines the target orbitals with which to maximize the overlap on each SCF cycle.

TYPE:

INTEGER

DEFAULT:

MOM

OPTIONS:

MOM
Maximize overlap with the orbitals from the previous SCF cycle.
IMOM
Maximize overlap with the initial guess orbitals.

RECOMMENDATION:

If appropriate guess orbitals can be obtained, then IMOM
can provide more reliable convergence to the desired solution.^{Barca:2018}

$molecule +1 1 C 0.000000 0.000000 0.990770 H 0.000000 0.000000 2.081970 C -1.233954 0.000000 0.290926 C -2.444677 0.000000 1.001437 H -2.464545 0.000000 2.089088 H -3.400657 0.000000 0.486785 C -1.175344 0.000000 -1.151599 H -2.151707 0.000000 -1.649364 C 0.000000 0.000000 -1.928130 C 1.175344 0.000000 -1.151599 H 2.151707 0.000000 -1.649364 C 1.233954 0.000000 0.290926 C 2.444677 0.000000 1.001437 H 2.464545 0.000000 2.089088 H 3.400657 0.000000 0.486785 $end $rem UNRESTRICTED false METHOD hf BASIS 6-31+G* SCF_GUESS core THRESH 12 $end @@@ $molecule read $end $rem UNRESTRICTED false METHOD hf BASIS 6-31+G* SCF_GUESS read MOM_START 1 THRESH 12 $end $occupied 1:26 28 1:26 28 $end @@@ $molecule -1 3 read $end $rem UNRESTRICTED false METHOD hf BASIS 6-31+G* SCF_GUESS read THRESH 12 $end