Next: Colvars as scripted functions Up: Collective variable components (basis Previous: Advanced usage and special   Contents   Index

## Linear and polynomial combinations of components

To extend the set of possible definitions of colvars , multiple components can be summed with the formula:

 (13.13)

where each component appears with a unique coefficient (1.0 by default) the positive integer exponent (1 by default).

Any set of components can be combined within a colvar, provided that they return the same type of values (scalar, unit vector, vector, or quaternion). By default, the colvar is the sum of its components. Linear or polynomial combinations (following equation (13.14)) can be obtained by setting the following parameters, which are common to all components:

• componentCoeff Coefficient of this component in the colvar
Context: any component
Acceptable values: decimal
Default value: 1.0
Description: Defines the coefficient by which this component is multiplied (after being raised to componentExp) before being added to the sum.

• componentExp Exponent of this component in the colvar
Context: any component
Acceptable values: integer
Default value: 1
Description: Defines the power at which the value of this component is raised before being added to the sum. When this exponent is different than 1 (non-linear sum), system forces and the Jacobian force are not available, making the colvar unsuitable for ABF calculations. Alternately, defines the order number of components within scripted colvars. Values should then range from 1 to the number of components. See 13.4.4 for details.

Example: To define the average of a colvar across different parts of the system, simply define within the same colvar block a series of components of the same type (applied to different atom groups), and assign to each component a componentCoeff of .

Next: Colvars as scripted functions Up: Collective variable components (basis Previous: Advanced usage and special   Contents   Index
vmd@ks.uiuc.edu