Crystallography & Symmetry
patterns derived from their groups
Concept

Rigidity matrix — where it appears

The matrix whose rows say how each bar of a framework changes length when the joints move. Its kernel is the framework's motions and its corank the number of them, computed exactly over the rationals when the phases are ±1.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

11 frameworks, 3 where the count is wrong. Every net in this collection read as a framework of rigid bars and free joints, with the cell free to change shape. Maxwell's count and the number of mechanisms agree on most of them and not on all: a framework with a state of self-stress has a bar the count treats as removing a freedom that the others had already removed, and it has a mechanism the count cannot see. Here that is fes, snb, ring5, where the count says 2, -1, -4 and the rank says 3, 0, 9. The identity Maxwell is always right about — count equals mechanisms minus self-stresses — holds on every row.

The count that promises a mechanism

Count the joints, count the bars, subtract. The number that comes out promises rigidity when it is small and a mechanism when it is large, and it is wrong in both directions — because it assumes every bar removes a freedom the others have not already removed.

applied · Rigidity the kagome net: 34 of 144 wavevectors carry a mechanism. The zone of the kagome net, with a mark at every wavevector whose rigidity matrix drops rank — which is to say at every wavevector that carries a motion of the bars. There are few of them and they are isolated, so enlarging the cell adds mechanisms slowly. The ranks at the half-integer wavevectors are exact; the others are computed with a stated tolerance, because the matrix there has genuinely complex entries.

A mechanism that is a wave

The framework essays found the kagome net's mechanism count growing with the cell it was looked for in, and recorded it as a finding without an explanation. Here is the explanation: the motions lie along lines in reciprocal space, and a larger cell samples a line at more places.

applied · Rigidity

Named alongside it

The objects these essays reach for when they reach for this one.

MechanismSelf stressDegrees of freedomGram matrixIsostaticKagome netMaxwell countPeriodic frameworkRigid unit modeSupercellWavevector

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