COMBINE
 This is part of the function module

Calculate a polynomial combination of a set of other variables.

The functional form of this function is

$C=\sum_{i=1}^{N_{arg}} c_i (x_i-a_i)^{p_i}$

The coefficients c, the parameters a and the powers p are provided as vectors.

Notice that COMBINE is not able to predict which will be periodic domain of the computed value automatically. The user is thus forced to specify it explicitly. Use PERIODIC=NO if the resulting variable is not periodic, and PERIODIC=A,B where A and B are the two boundaries if the resulting variable is periodic.

Compulsory keywords
 PERIODIC if the output of your function is periodic then you should specify the periodicity of the function. If the output is not periodic you must state this using PERIODIC=NO COEFFICIENTS ( default=1.0 ) the coefficients of the arguments in your function PARAMETERS ( default=0.0 ) the parameters of the arguments in your function POWERS ( default=1.0 ) the powers to which you are raising each of the arguments in your function
Options
 NUMERICAL_DERIVATIVES ( default=off ) calculate the derivatives for these quantities numerically NORMALIZE ( default=off ) normalize all the coefficients so that in total they are equal to one 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...
Examples

The following input tells plumed to print the distance between atoms 3 and 5 its square (as computed from the x,y,z components) and the distance again as computed from the square root of the square.

DISTANCE LABEL=dist      ATOMS=3,5 COMPONENTS
COMBINE  LABEL=distance2 ARG=dist.x,dist.y,dist.z POWERS=2,2,2 PERIODIC=NO
COMBINE  LABEL=distance  ARG=distance2 POWERS=0.5 PERIODIC=NO
PRINT ARG=distance,distance2


t: TORSION ATOMS=1,3,5,7