diff --git a/src/generic/Group.cpp b/src/generic/Group.cpp
index 8c77fde1b7e8b2015dc5e9599b37f529723e1faa..ecd562e8ff3855832b4f3d416ebe298e34fdc478 100644
--- a/src/generic/Group.cpp
+++ b/src/generic/Group.cpp
@@ -56,6 +56,7 @@ The flow is the following:
 - If UNIQUE is present the resuling list is sorted and duplicate elements are removed.
 
 Notice that this command just creates a shortcut, and does not imply any real calculation.
+So, having a huge group defined does not slow down your calculation in any way.
 It is just convenient to better organize input files. Might be used in combination with
 the \ref INCLUDE command so as to store long group definitions in a separate file.
 
diff --git a/user-doc/Functions.txt b/user-doc/Functions.txt
index dc6aafd01b4161ed94b24cbac46bff522dc4f127..8bf37c42db0e66f5e2727bb98b419edeb4f88217 100644
--- a/user-doc/Functions.txt
+++ b/user-doc/Functions.txt
@@ -3,7 +3,43 @@
 
 When performing biased dynamics or analysing a trajectory you may wish to analyse/bias the value of
 some function of a set of collective variables rather than the values of the collective variables
-directly.  You can do this with PLUMED by using any one of the following list of functions: 
+directly.  You can do this with PLUMED by using any one of the following list of functions.
+
+Notice that in many functions you should explicitly say to PLUMED whether the result
+is a periodic variable or not using the keyword `PERIODIC`.
+This is crucial to allow a variable to be properly based.
+To know if a function is periodic
+of not you should answer to the following question:
+
+- Can my function change with a discontinuity when I move my atoms in a continuous manner?
+
+In case the answer is no, than you should use `PERIODIC=NO`. In case the answer is yes, then you should
+consider the following question:
+
+- Are the values of the function at the discontinuity always the same or do they change?
+
+In case the answer is that they are the same, you should use `PERIODIC=A,B` where `A`
+is the smallest value and `B` is the largest value. In case the answer is that the
+values at the discontinuity are not always the same, then you cannot construct a variable that
+can be biased with PLUMED. Consider the following examples:
+
+\verbatim
+t: TORSION ATOMS=1,2,3,4
+# When atoms are moved, t could jump suddenly from -pi to +pi
+
+c: MATHEVAL ARG=t FUNC=x*x*x PERIODIC=-31.0062766802998,31.0062766802998
+# When atoms are moved, c could jump suddenly from -pi**3 to +pi**3
+
+# equivalently, we could have used:
+# c: COMBINE ARG=t POWERS=3 PERIODIC=-31.0062766802998,31.0062766802998
+
+# compute x/y/z components of the distance between atoms 1 and 10
+d: DISTANCE ATOMS=1,10 COMPONENTS
+
+# make a new variable equal to d.z but with the correct periodicity
+dz: COMBINE ARG=d.z PERIODIC=-10,10
+# here we assumed the system is in a orthorhombic box with z side = 20
+\endverbatim
 
 @FUNCTION@
 
diff --git a/user-doc/Misc.txt b/user-doc/Misc.txt
index 0ee019d2695eb53ae2081fcc4d76e3c2f132e52f..284a2a6e9bdfc65ce0b7e12f698bafd089cea19d 100644
--- a/user-doc/Misc.txt
+++ b/user-doc/Misc.txt
@@ -281,12 +281,13 @@ With these settings, in the first example (`DU<CTRL+X><CTRL+O`) VIM will only co
 
 As you can imagine,
 if you use autocompletion after you have typed the word `DISTANCE` followed by a space you will see
-a menu listing `LABEL=`, `COMPONENTS`, etc. Basically, all the keywords that are possibly used withint a `DISTANCE` line
-will be shown.
+a menu listing `LABEL=`, `COMPONENTS`, etc. Basically, all the keywords that are possibly used within a `DISTANCE` line
+will be shown. This is very useful if you do not remember the exact name of the keywords associated with
+a given action.
 
 \par Quick help
 
-You can also retrieve quick explanation of the input options for a specific collective variable.
+You can also retrieve quick explanation of the input options for a specific action.
 Try to do the following. Enable plumed syntax:
 \verbatim
 :set ft=plumed
@@ -328,7 +329,7 @@ To simplify this, when PLUMED syntax for VIM is configured properly VIM should b
 Notice that the syntax file for the output files (`plumedf.vim`) is not the same one that is used for the PLUMED
 input file (`plumed.vim`).
 
-To make output files more readable, vi will show `FIELDS` and `SET` words in a different color,
+To make output files more readable, vim will show `FIELDS` and `SET` words in a different color,
 and data columns with alternating colors (e.g. dark/light/dark/light).
 The colors in the columns are consistent with those shown in the FIELD line.
 In the example above, 1, 2, and 3 will be of the same color as A, B, and C respectively.