The LOWER_WALLS and UPPER_WALLS keywords define a wall for the value of one or more collective variables, which limits the region of the phase space accessible during the simulation.

The restraining potential starts acting on the system when the value of the CV is greater (in the case of UPPER_WALLS) or lower (in the case of LOWER_WALLS) than a certain limit \(a_i\) (AT) minus an offset \(o_i\) (OFFSET). The expression for the bias due to the wall is given by:

\( \sum_i {k_i}((x_i-a_i+o_i)/s_i)^e_i \)

\(k_i\) (KAPPA) is an energy constant in internal unit of the code, \(s_i\) (EPS) a rescaling factor and \(e_i\) (EXP) the exponent determining the power law. By default: EXP = 2, EPS = 1.0, OFF = 0.

Description of components

By default this Action calculates the following quantities. These quanties can be referenced elsewhere in the input by using this Action's label followed by a dot and the name of the quantity required from the list below.

Quantity	Description
bias	the instantaneous value of the bias potential
force2	the instantaneous value of the squared force due to this bias potential

Compulsory keywords

ARG	the input for this action is the output from one or more other actions. The particular output that you used is referenced using that action of interests label. If the label appears on its own then the value of the relevant Action is taken. If * or . appears the information from all arguments is taken. Some actions have multi-component outputs, each component of the output has a specific label so for instance an action labelled dist may have three componets x, y and z. To take just the x component you should use dist.x, if you wish to take all three components then use dist.*
AT	the positions of the wall. The a_i in the expression for a wall.
KAPPA	the force constant for the wall. The k_i in the expression for a wall.
OFFSET	( default=0.0 ) the offset for the start of the wall. The o_i in the expression for a wall.
EXP	( default=2.0 ) the powers for the walls. The e_i in the expression for a wall.
EPS	( default=1.0 ) the values for s_i in the expression for a wall

Options

NUMERICAL_DERIVATIVES

( default=off ) calculate the derivatives for these quantities numerically

Examples

The following input tells plumed to add both a lower and an upper walls on the distance between atoms 3 and 5 and the distance between atoms 2 and 4. The lower and upper limits are defined at different values. The strength of the walls is the same for the four cases. It also tells plumed to print the energy of the walls.

DISTANCE ATOMS=3,5 LABEL=d1
DISTANCE ATOMS=2,4 LABEL=d2
UPPER_WALLS ARG=d1,d2 AT=1.0,1.5 KAPPA=150.0,150.0 EXP=2,2 EPS=1,1 OFFSET 0,0 LABEL=uwall
LOWER_WALLS ARG=d1,d2 AT=0.0,1.0 KAPPA=150.0,150.0 EXP=2,2 EPS=1,1 OFFSET 0,0 LABEL=lwall
PRINT ARG=uwall.bias,lwall.bias

(See also DISTANCE and PRINT).