Chain translate

Keyword:

MOVE_CHAIN_TRANSLATE

Move code:

2

Status:

core

How it works

The whole chain is moved as a rigid body by a single random displacement vector: a per-dimension offset is drawn uniformly and every bead of the chain is shifted by it (with periodic wrapping). If any translated bead would land on an occupied site the move is rejected as a hard-sphere clash; under HARDWALL a chain that would straddle the boundary is also rejected. The chain’s internal conformation is unchanged - only its position in the box moves.

This is one of the more expensive single-chain moves, because the energy of every bead has to be re-evaluated against its new surroundings, but it is the primary way an intact chain explores different regions of the box.

Why detailed balance holds

The displacement is drawn uniformly, and on the lattice a shift by \(+v\) and the reverse shift by \(-v\) are equally likely, so the proposal is symmetric,

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

The move is therefore accepted with the plain Metropolis criterion \(A = \min(1, e^{-\Delta E/T})\), which satisfies detailed balance (see the primer). Hard-sphere clashes correspond to \(\pi = 0\) states and are rejected with certainty, consistent with balance.

Configuration

MOVE_CHAIN_TRANSLATEfloat

Probability of selecting a chain-translation step (all MOVE_* must sum to 1.0).

There are no other tuning keywords. In dense systems most translations clash and are rejected; for relocating chains through crowded/condensed phases prefer the collective moves (Virtual-Move Monte Carlo (VMMC), Pull (cooperative reptation)) instead.