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MoleculesMolecules are composed of several atoms = particlesand each is subjected to forces. One can bind themtogether through the attractive part of their potentialbut that needs deep potentials to avoid them fallingapart and therefore is computationally expensive.Since forces inside molecules are at least one orderof magnitude larger than those between them onecan just keep bonds (and angles) fixed.37MoleculesThere exist several ways to implement fixeddistances (or angles) inside a molecule or anyother structure that couples degrees offreedom:• Imposing constraints using Lagrangemultipliers.• Describing structures as rigid bodies.38