LOWER_WALLS
 This is part of the bias module

Defines 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, OFFSET = 0.

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.

Click on the labels of the actions for more information on what each action computes
d1: DISTANCE ATOMSthe pair of atom that we are calculating the distance between. =3,5
d2: DISTANCE ATOMSthe pair of atom that we are calculating the distance between. =2,4
uwall: UPPER_WALLS ARGthe input for this action is the scalar output from one or more other actions. =d1,d2 ATcompulsory keyword
the positions of the wall. =1.0,1.5 KAPPAcompulsory keyword
the force constant for the wall. =150.0,150.0 EXPcompulsory keyword ( default=2.0 )
the powers for the walls. =2,2 EPScompulsory keyword ( default=1.0 )
the values for s_i in the expression for a wall =1,1 OFFSETcompulsory keyword ( default=0.0 )
the offset for the start of the wall. =0,0
lwall: LOWER_WALLS ARGthe input for this action is the scalar output from one or more other actions. =d1,d2 ATcompulsory keyword
the positions of the wall. =0.0,1.0 KAPPAcompulsory keyword
the force constant for the wall. =150.0,150.0 EXPcompulsory keyword ( default=2.0 )
the powers for the walls. =2,2 EPScompulsory keyword ( default=1.0 )
the values for s_i in the expression for a wall =1,1 OFFSETcompulsory keyword ( default=0.0 )
the offset for the start of the wall. =0,0
PRINT ARGthe input for this action is the scalar output from one or more other actions. =uwall.bias,lwall.bias


Glossary of keywords and components
Description of components

By default this Action calculates the following quantities. These quantities 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
 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 ARG the input for this action is the scalar output from one or more other actions. The particular scalars that you will use are referenced using the label of the action. If the label appears on its own then it is assumed that the Action calculates a single scalar value. The value of this scalar is thus used as the input to this new action. If * or *.* appears the scalars calculated by all the proceeding actions in the input file are taken. Some actions have multi-component outputs and each component of the output has a specific label. For example a DISTANCE action labelled dist may have three components 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.*.More information on the referencing of Actions can be found in the section of the manual on the PLUMED Getting Started. Scalar values can also be referenced using POSIX regular expressions as detailed in the section on Regular Expressions. To use this feature you you must compile PLUMED with the appropriate flag. You can use multiple instances of this keyword i.e. ARG1, ARG2, ARG3...