OpenMM

MTSIntegrator implements the rRESPA multiple time step integration algorithm. More...
Inherits CustomIntegrator.
Public Member Functions  
def  __init__ 
Create an MTSIntegrator. 
MTSIntegrator implements the rRESPA multiple time step integration algorithm.
This integrator allows different forces to be evaluated at different frequencies, for example to evaluate the expensive, slowly changing forces less frequently than the inexpensive, quickly changing forces.
To use it, you must first divide your forces into two or more groups (by calling setForceGroup() on them) that should be evaluated at different frequencies. When you create the integrator, you provide a tuple for each group specifying the index of the force group and the frequency (as a fraction of the outermost time step) at which to evaluate it. For example:
integrator = MTSIntegrator(4*femtoseconds, [(0,1), (1,2), (2,8)])
This specifies that the outermost time step is 4 fs, so each step of the integrator will advance time by that much. It also says that force group 0 should be evaluated once per time step, force group 1 should be evaluated twice per time step (every 2 fs), and force group 2 should be evaluated eight times per time step (every 0.5 fs).
A common use of this algorithm is to evaluate reciprocal space nonbonded interactions less often than the bonded and direct space nonbonded interactions. The following example looks up the NonbondedForce, sets the reciprocal space interactions to their own force group, and then creates an integrator that evaluates them once every 4 fs, but all other interactions every 2 fs.
nonbonded = [f for f in system.getForces() if isinstance(f, NonbondedForce)][0] nonbonded.setReciprocalSpaceForceGroup(1) integrator = MTSIntegrator(4*femtoseconds, [(1,1), (0,2)])
For details, see Tuckerman et al., J. Chem. Phys. 97(3) pp. 19902001 (1992).
def __init__  (  self,  
dt,  
groups  
) 
Create an MTSIntegrator.
dt  (time) The largest (outermost) integration time step to use 
groups  (list) A list of tuples defining the force groups. The first element of each tuple is the force group index, and the second element is the number of times that force group should be evaluated in one time step. 