UPDATE_IF
 This is part of the generic module

Conditional update of other actions.

This action can be used to enable and disable the update step for the following actions depending on the value of its arguments. This allows for example to extract snapshots with value of some CVs in a given range.

When called with MORE_THAN and/or LESS_THAN keywords, this action starts an if block. The block is executed if all the arguments are less than all the respective values in the LESS_THAN keyword (if present) and all the arguments are more than all the respective values in the MORE_THAN keyword (if present).

When called with the END flag, this action ends the corresponding IF block. Notice that in this case one should also provide the ARG keyword. It is recommended to use the same ARG keyword that was used to begin the block, so as to make the input more readable.

Of course, blocks can be nested at will.

There are many potential usages for this keyword. One might e.g. decide to analyze some variable only when another variable is within a given range.

Warning
Notice that not all the possible usage make particular sense. For example, conditionally updating a METAD keyword (that is: adding hills only if a variable is within a given range) can lead to unexpected results.
Compulsory keywords
 STRIDE ( default=1 ) the frequency with which the quantities of interest should be output
Options
 END ( default=off ) end 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 proceding 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 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.*.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... LESS_THAN upper bound MORE_THAN lower bound
Examples

The following input instructs plumed dump all the snapshots where an atom is in touch with the solute.

solute: GROUP ATOMS=1-124
coord: COORDINATION GROUPA=solute GROUPB=500 R_0=0.5

# A coordination number higher than 0.5 indicate that there is at least one
# atom of group solute at less than 5 A from atom number 500

UPDATE_IF ARG=coord MORE_THAN=0.5
DUMPATOMS ATOMS=solute,500 FILE=output.xyz
UPDATE_IF ARG=coord END