*Accepted paper*

**Inserted:** 30 jan 2017

**Last Updated:** 24 apr 2017

**Journal:** J. Nonlinear Sci.

**Year:** 2017

**Abstract:**

We show that the emerging field of discrete differential geometry can be usefully brought to bear on crystallization problems. In particular, we give a simplified proof of the Heitmann-Radin crystallization theorem \cite{HR}, which concerns a system of $N$ identical atoms in two dimensions interacting via the idealized pair potential $V(r)=+\infty$ if $r<1$, $-1$ if $r=1$, $0$ if $r>1$. This is done by endowing the bond graph of a general particle configuration with a suitable notion of {\it discrete curvature}, and appealing to a {\it discrete Gauss-Bonnet theorem} \cite{Knill1} which, as its continuous cousins, relates the sum*integral of the curvature to topological invariants. This leads to an exact geometric decomposition of the Heitmann-Radin energy into (i) a combinatorial bulk term, (ii) a combinatorial perimeter, (iii) a multiple of the Euler characteristic, and (iv) a natural topological energy contribution due to defects. An analogous exact geometric decomposition is also established for soft potentials such as the Lennard-Jones potential $V(r)=r^{-6}-2r^{-12}$, where two additional contributions arise, (v) elastic energy and (vi) energy due to non-bonded interactions. *

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