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The Neo-Hookean hyperelastic solid constitutive model.[^1] **Parameters** - The bulk modulus $`\kappa`$. - The shear modulus $`\mu`$. **External variables** - The deformation gradient $`\mathbf{F}`$. **Internal variables** - None. [^1]: R.S. Rivlin, [Philos. Trans. R. Soc. London, Ser. A **240**, 459 (1948)](https://doi.org/10.1098/rsta.1948.0002).