Chain pivot

Keyword:

MOVE_CHAIN_PIVOT

Move code:

4

Status:

core

How it works

A pivot move chooses a random interior bead and rigidly rotates the whole portion of the chain on one side of it (a cardinal lattice rotation) about that pivot point, leaving the other side fixed. Because it moves a large, contiguous section of the chain in one shot, a pivot makes much larger conformational changes than a crankshaft - it is an efficient way to decorrelate the global shape of a chain, especially for swollen/dilute chains. A clash of any moved bead (or a hardwall straddle) rejects the move.

Why detailed balance holds

The pivot point is chosen uniformly along the chain and the rotation uniformly from the cardinal rotations; both choices, and their inverses, are equally likely for the reverse move, so the proposal is symmetric,

\[g(x\to y) = g(y\to x),\]

and the move uses the plain Metropolis acceptance \(A = \min(1, e^{-\Delta E/T})\) (see the primer).

Configuration

MOVE_CHAIN_PIVOTfloat

Probability of selecting a pivot step (all MOVE_* must sum to 1.0).

Pivots have a low acceptance rate in dense systems (the rotated tail usually clashes) but are very effective for single chains and dilute solutions, where a modest fraction substantially speeds up sampling of the overall chain dimensions.