# Specialist and rarely-used keywords¶

## GRAVITY¶

This grid section keyword allows to the user to change the gravity vector from its default value. However if the user attempts to initialize the problem using hydrostatic equilibration, and a non-default gravity value, the code will return an error.

• GRAVITY <gx gy gz>: Specifies gravity vector components (default: 0. 0. -9.8068)

See example below, where the gravity effects are neglected

GRID
TYPE structured
ORIGIN 0.d0 0.d0 0.d0
GRAVITY 0.d0 0.d0 0.d0
NXYZ 500 1 100
BOUNDS
0.d0 0.0 0.0
250.d0 1.d0 50.d0
/
END


## GEOMETRIC_PENALTY¶

This is only valid if mode is TOIL_IMS. If present, the following heuristic is executed in the nonlinear solver: if an update from the nonlinear solver would change a saturation value in a cell by more than 20% of its original value, then the update of that saturation value is set to change the value by 20%. Like most heuristics, it is not useful in every situation or model.

## MAX_PRESSURE_CHANGE¶

PFLOTRAN will aim to have no more than this value as the change in pressure each timestep, adapting the timestep length accordingly. In effect after a timestep where the pressure change exceeded this value, the timestep length will decrease, after a timestep where the pressure change was less than this value, the timestep length may increase if permitted by other factors. The default value is 5.0d5 Pa.