This is part of the function module

Calculates the replica averaging of a collective variable over multiple replicas.

Each collective variable is averaged separately and stored in a component labelled label.cvlabel.

Note that in case of variables such as CS2BACKBONE, CH3SHIFTS, NOE and RDC it is possible to perform the replica-averaging inside the variable, in fact in those cases are the single experimental values that averaged before calculating the collective variable.

Compulsory keywords
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.

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

The following input tells plumed to calculate the distance between atoms 3 and 5 and the average it over the available replicas.
ens: ENSEMBLE ARG=dist
PRINT ARG=dist,ens.dist
(See also PRINT and DISTANCE).