HMOC - Hybrid Modified Method of Characteristics mixes MOC and MMOC based on specified criteria:

When a sharp front is present, the advection term is solved by MOC using packets dynamically distributed around the front

Away from that front advection is solved by MMOC using packets placed at nodes and tracked backwards.

When the front dissipates (due to dispersion, reactions, mixing) the forward tracking stops automatically and the packets are removed
If control parameters for switching between MOC and MMOC are set correctly, you can get accurate solutions for all Peclet numbers with no dispersion

If the flow field and dispersivity values are fairly constant and the gridding is fairly regular you can select MOC or MMOC based on the grid Peclet #
as a RULE OF THUMB, BUT ONLY THAT
Pe > 10 use MOC
PE< 0.1 use MMOC
When MOC requires too much computer time and MMOC introduces "noticeable" numerical dispersion HMOC may be best

When HMOC is used a parameter DCHMOC is defined by the modeler and compared to:
The relative cell concentration gradient DCCELL which indicates the "sharpness" of the plume front near each cell by comparing the difference in concentration in the vicinity of the cell with the difference over the entire grid.

CMAXi,j,k - maximum concentration near the cell
CMINi,j,k - minimum concentration near the cell
CMAX - maximum concentration in the entire grid
CMIN - minimum concentration in the entire grid

So if CMAX-CMIN for the entire grid is 10000 and near the node of interest the difference is 8000, that is a sharp front (0.8) and MOC should be used whereas, if the difference near the node is 10 it is not a sharp front (0.001) and MMOC can be used
when DCCELL > DCHMOC - MOC IS USED
when DCCELL < DCHMOC - MMOC IS USED

DCHMOC value is defined by the user and it has been empirically determined that 0.001 to 0.01 are generally appropriate values

Sometimes it is difficult to find the right switching parameters and it is better to just use MOC or MMOC
- Also, when feasible, it is best to work with Peclet numbers of 0 to 2