Multiple time stepping

By setting a STRIDE different from 1, you change how frequently an action is calculated. In the case of actions such as PRINT, this just means how frequently you dump some quantity on the disk. Notice that variables are only computed when necessary. Thus, if a variable is only appearing as the argument of a PRINT statement with STRIDE=10, it will be computed every 10 steps.

In a similar fashion, the STRIDE keyword can be used in a bias potential so as to apply the bias potential every few steps. In this case, forces from this bias potential are scaled up by a factor equal to STRIDE.

This technique can allow your simulation to run faster if you need the apply a bias potential on some very expensive collective variable. Consider the following input:

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

c1: COM 
ATOMS
the list of atoms which are involved the virtual atom's definition. 
=1-1000 The COM action with label c1 calculates the position of a virtual atom
c2: COM 
ATOMS
the list of atoms which are involved the virtual atom's definition. 
=1001-2000 The COM action with label c2 calculates the position of a virtual atom
d: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between. 
=c1,c2 The DISTANCE action with label d calculates a single scalar value
METAD 
ARG
the input for this action is the scalar output from one or more other actions. 
=d 
HEIGHT
the heights of the Gaussian hills. 
=1 
SIGMA
compulsory keyword 
the widths of the Gaussian hills 
=0.1 
BIASFACTOR
use well tempered metadynamics and use this bias factor. 
=5 
PACE
compulsory keyword 
the frequency for hill addition 
=500 The METAD action with label 

This performs a METAD simulation biasing the distance between two centers of mass. Since computing these centers requires a lot of atoms to be imported from the MD engine, it could slow down significantly the simulation. Notice that whereas the bias is changed every PACE=500 steps, it is applied every STRIDE step, where STRIDE=1 by default. The following input could lead to a significantly faster simulation at the price of a negligible systematic error

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

c1: COM 
ATOMS
the list of atoms which are involved the virtual atom's definition. 
=1-1000 The COM action with label c1 calculates the position of a virtual atom
c2: COM 
ATOMS
the list of atoms which are involved the virtual atom's definition. 
=1001-2000 The COM action with label c2 calculates the position of a virtual atom
d: DISTANCE 
ATOMS
the pair of atom that we are calculating the distance between. 
=c1,c2 The DISTANCE action with label d calculates a single scalar value
METAD 
ARG
the input for this action is the scalar output from one or more other actions. 
=d 
HEIGHT
the heights of the Gaussian hills. 
=1 
SIGMA
compulsory keyword 
the widths of the Gaussian hills 
=0.1 
BIASFACTOR
use well tempered metadynamics and use this bias factor. 
=5 
PACE
compulsory keyword 
the frequency for hill addition 
=500 
STRIDE
the frequency with which the forces due to the bias should be calculated. 
=2 The METAD action with label 

Similarly, the STRIDE keyword can be used with other biases (e.g. RESTRAINT).

The technique is discussed in details here [50]. See also EFFECTIVE_ENERGY_DRIFT.