This is part of the multicolvar module

This action can be used to filter the distribution of colvar values in a multicolvar so that one can compute the mean and so on for only those multicolvars more than a tolerance.

This action can be used to create a dynamic group of atom based on the value of a multicolvar. In this action a multicolvar is within the dynamic group if its value is greater than a target. In actuality a weight, \(w_i\) is ascribed to each colvar, \(s_i\) calculated by a multicolvar and this weight measures the degree to which a colvar is a member of the group. This weight is calculated using a switchingfunction , \(\sigma\) so it is given by:

\[ w_i = 1 - \sigma(s_i) \]

If one calculates a function of the set of multicolvars these weights are included in the calculation. As such if one calculates the MEAN, \(\mu\) of a filtered multicolvar what is computed is the following:

\[ \mu = \frac{ \sum_i w_i s_i }{ \sum_i w_i} \]

One is thus calculating the mean for those colvars that are greater than the target.

Examples

The example shown below calculates the mean for those distances that greater than 1.5 nm in length

Click on the labels of the actions for more information on what each action computes

d1: DISTANCES GROUPACalculate the distances between all the atoms in GROUPA and all the atoms in GROUPB.
=1 GROUPBCalculate the distances between all the atoms in GROUPA and all the atoms in GROUPB.
=2-50 MEANtake the mean of these variables.  The DISTANCES action with label d1 calculates a single scalar value
d4: MFILTER_MORE DATAcompulsory keyword 
The multicolvar that calculates the set of base quantities that we are interested
in =d1 SWITCHThis keyword is used if you want to employ an alternative to the continuous switching
function defined above. ={GAUSSIAN D_0=1.5 R_0=0.00001}  MEANtake the mean of these variables.  The MFILTER_MORE action with label d4 calculates a single scalar value

More complicated things can be done by using the label of a filter as input to a new multicolvar as shown in the example below. Here the coordination numbers of all atoms are computed. The atoms with a coordination number greater than 2 are then identified using the filter. This reduced list of atoms is then used as input to a second coordination number calculation. This second coordination number thus measures the number of two-coordinated atoms that each of the two-coordinated atoms is bound to.

Click on the labels of the actions for more information on what each action computes

c1: COORDINATIONNUMBER SPECIESthis keyword is used for colvars such as coordination number. =1-150 SWITCHThis keyword is used if you want to employ an alternative to the continuous switching
function defined above. ={EXP D_0=4.0 R_0=0.5 D_MAX=6.0}  The COORDINATIONNUMBER action with label c1
cf: MFILTER_MORE DATAcompulsory keyword 
The multicolvar that calculates the set of base quantities that we are interested
in =c1 SWITCHThis keyword is used if you want to employ an alternative to the continuous switching
function defined above. ={RATIONAL D_0=2.0 R_0=0.1}  LOWMEM( default=off ) lower the memory requirements  The MFILTER_MORE action with label cf
c2: COORDINATIONNUMBER SPECIESthis keyword is used for colvars such as coordination number. =cf SWITCHThis keyword is used if you want to employ an alternative to the continuous switching
function defined above. ={EXP D_0=4.0 R_0=0.5 D_MAX=6.0}  MORE_THANcalculate the number of variables more than a certain target value. ={RATIONAL D_0=2.0 R_0=0.1}  The COORDINATIONNUMBER action with label c2 calculates a single scalar value

Glossary of keywords and components

Description of components

When the label of this action is used as the input for a second you are not referring to a scalar quantity as you are in regular collective variables. The label is used to reference the full set of quantities calculated by the action. This is usual when using MultiColvar functions. Generally when doing this the previously calculated multicolvar will be referenced using the DATA keyword rather than ARG.

This Action can be used to calculate the following scalar quantities directly. These quantities are calculated by employing the keywords listed below. These quantities can then be referenced elsewhere in the input file by using this Action's label followed by a dot and the name of the quantity. Some of them can be calculated multiple times with different parameters. In this case the quantities calculated can be referenced elsewhere in the input by using the name of the quantity followed by a numerical identifier e.g. label.lessthan-1, label.lessthan-2 etc. When doing this and, for clarity we have made it so that the user can set a particular label for each of the components. As such by using the LABEL keyword in the description of the keyword input you can customize the component name

Quantity	Keyword	Description
vmean	VMEAN	the norm of the mean vector. The output component can be referred to elsewhere in the input file by using the label.vmean
altmin	ALT_MIN	the minimum value. This is calculated using the formula described in the description of the keyword so as to make it continuous.
highest	HIGHEST	the highest of the quantities calculated by this action
lowest	LOWEST	the lowest of the quantities calculated by this action
max	MAX	the maximum value. This is calculated using the formula described in the description of the keyword so as to make it continuous.
mean	MEAN	the mean value. The output component can be referred to elsewhere in the input file by using the label.mean
min	MIN	the minimum value. This is calculated using the formula described in the description of the keyword so as to make it continuous.
moment	MOMENTS	the central moments of the distribution of values. The second moment would be referenced elsewhere in the input file using label.moment-2, the third as label.moment-3, etc.

Compulsory keywords

DATA	The multicolvar that calculates the set of base quantities that we are interested in
NN	( default=6 ) The n parameter of the switching function
MM	( default=0 ) The m parameter of the switching function; 0 implies 2*NN
D_0	( default=0.0 ) The d_0 parameter of the switching function
R_0	The r_0 parameter of the switching function

Options

NUMERICAL_DERIVATIVES	( default=off ) calculate the derivatives for these quantities numerically
NOPBC	( default=off ) ignore the periodic boundary conditions when calculating distances
SERIAL	( default=off ) do the calculation in serial. Do not use MPI
LOWMEM	( default=off ) lower the memory requirements
TIMINGS	( default=off ) output information on the timings of the various parts of the calculation
VMEAN	calculate the norm of the mean vector. The final value can be referenced using label.vmean. You can use multiple instances of this keyword i.e. VMEAN1, VMEAN2, VMEAN3... The corresponding values are then referenced using label.vmean-1, label.vmean-2, label.vmean-3...
MEAN	take the mean of these variables. The final value can be referenced using label.mean. You can use multiple instances of this keyword i.e. MEAN1, MEAN2, MEAN3... The corresponding values are then referenced using label.mean-1, label.mean-2, label.mean-3...
MOMENTS	calculate the moments of the distribution of collective variables. The mth moment of a distribution is calculated using \(\frac{1}{N} \sum_{i=1}^N ( s_i - \overline{s} )^m \), where \(\overline{s}\) is the average for the distribution. The moments keyword takes a lists of integers as input or a range. Each integer is a value of \(m\). The final calculated values can be referenced using moment- \(m\). You can use the COMPONENT keyword in this action but the syntax is slightly different. If you would like the second and third moments of the third component you would use MOMENTS={COMPONENT=3 MOMENTS=2-3}. The moments would then be referred to using the labels moment-3-2 and moment-3-3. This syntax is also required if you are using numbered MOMENT keywords i.e. MOMENTS1, MOMENTS2...
MIN	calculate the minimum value. To make this quantity continuous the minimum is calculated using \( \textrm{min} = \frac{\beta}{ \log \sum_i \exp\left( \frac{\beta}{s_i} \right) } \) The value of \(\beta\) in this function is specified using (BETA= \(\beta\)) The final value can be referenced using label.min. You can use multiple instances of this keyword i.e. MIN1, MIN2, MIN3... The corresponding values are then referenced using label.min-1, label.min-2, label.min-3...
MAX	calculate the maximum value. To make this quantity continuous the maximum is calculated using \( \textrm{max} = \beta \log \sum_i \exp\left( \frac{s_i}{\beta}\right) \) The value of \(\beta\) in this function is specified using (BETA= \(\beta\)) The final value can be referenced using label.max. You can use multiple instances of this keyword i.e. MAX1, MAX2, MAX3... The corresponding values are then referenced using label.max-1, label.max-2, label.max-3...
ALT_MIN	calculate the minimum value. To make this quantity continuous the minimum is calculated using \( \textrm{min} = -\frac{1}{\beta} \log \sum_i \exp\left( -\beta s_i \right) \) The value of \(\beta\) in this function is specified using (BETA= \(\beta\)). The final value can be referenced using label.altmin. You can use multiple instances of this keyword i.e. ALT_MIN1, ALT_MIN2, ALT_MIN3... The corresponding values are then referenced using label.altmin-1, label.altmin-2, label.altmin-3...
LOWEST	this flag allows you to recover the lowest of these variables. The final value can be referenced using label.lowest
HIGHEST	this flag allows you to recover the highest of these variables. The final value can be referenced using label.highest
SWITCH	This keyword is used if you want to employ an alternative to the continuous switching function defined above. The following provides information on the switchingfunction that are available. When this keyword is present you no longer need the NN, MM, D_0 and R_0 keywords.