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+"""
+Neo-Hookean hyperelastic material model using Tensors.jl and automatic differentiation.
+
+This module implements the simplest hyperelastic material model derived from
+strain energy density. Uses ForwardDiff.jl for automatic computation of stress
+and tangent modulus from the strain energy function.
+
+Theory:
+    ψ(C) = μ/2·(I₁ - 3) - μ·ln(J) + λ/2·ln²(J)
+    
+Where:
+    C = FᵀF              Right Cauchy-Green tensor
+    I₁ = tr(C)           First invariant
+    J = √det(C)          Volume ratio (Jacobian)
+    μ, λ                 Lamé parameters
+
+Second Piola-Kirchhoff stress (energy conjugate to Green-Lagrange strain):
+    S = 2·∂ψ/∂C = μ(I - C⁻¹) + λ·ln(J)·C⁻¹
+
+Material tangent (Total Lagrangian formulation):
+    𝔻 = 4·∂²ψ/∂C²
+
+Key feature: Automatic differentiation eliminates manual derivative errors!
+"""
+
+using Tensors
+# Note: Tensors.jl provides hessian() function for automatic differentiation
+# No need for ForwardDiff.jl dependency!
+
+# Load abstract types
+include("abstract_material.jl")
+
+"""
+    NeoHookean <: AbstractElasticMaterial
+
+Neo-Hookean hyperelastic material model.
+
+Uses compressible Neo-Hookean strain energy with automatic differentiation
+for stress and tangent computation.
+
+# Fields
+- `μ::Float64` - Shear modulus [Pa]
+- `λ::Float64` - Lamé parameter [Pa] (controls compressibility)
+
+# Properties
+- Stateless material (no history dependence)
+- Geometrically nonlinear (finite strain)
+- Uses Total Lagrangian formulation (2nd PK stress)
+- Automatic differentiation for derivatives
+
+# Type Hierarchy
+`NeoHookean <: AbstractElasticMaterial <: AbstractMaterial`
+
+# Construction
+```julia
+# From Lamé parameters (direct)
+rubber = NeoHookean(μ=1e6, λ=1e9)
+
+# From engineering constants (convenience)
+rubber = NeoHookean(E=3e6, ν=0.45)
+```
+
+# Notes
+For nearly incompressible materials (ν → 0.5), use large λ relative to μ.
+"""
+struct NeoHookean <: AbstractElasticMaterial
+    μ::Float64  # Shear modulus [Pa]
+    λ::Float64  # Lamé parameter [Pa]
+
+    function NeoHookean(μ::Float64, λ::Float64)
+        μ > 0.0 || throw(ArgumentError("Shear modulus μ must be positive, got μ = $μ"))
+        λ > 0.0 || throw(ArgumentError("Lamé parameter λ must be positive, got λ = $λ"))
+        new(μ, λ)
+    end
+end
+
+"""
+    NeoHookean(; μ, λ)
+
+Convenience constructor with keyword arguments (Lamé parameters).
+
+# Example
+```julia
+rubber = NeoHookean(μ=1e6, λ=1e9)
+```
+"""
+function NeoHookean(; μ::Real=NaN, λ::Real=NaN, E_mod::Real=NaN, nu::Real=NaN)
+    # Check which set of parameters was provided
+    if !isnan(μ) && !isnan(λ)
+        # Lamé parameters provided
+        return NeoHookean(Float64(μ), Float64(λ))
+    elseif !isnan(E_mod) && !isnan(nu)
+        # Engineering constants provided
+        E_mod > 0.0 || throw(ArgumentError("Young's modulus E_mod must be positive, got E_mod = $E_mod"))
+        -1.0 < nu < 0.5 || throw(ArgumentError("Poisson's ratio must satisfy -1 < nu < 0.5, got nu = $nu"))
+
+        μ_val = E_mod / (2(1 + nu))
+        λ_val = E_mod * nu / ((1 + nu) * (1 - 2nu))
+
+        return NeoHookean(Float64(μ_val), Float64(λ_val))
+    else
+        throw(ArgumentError("Must provide either (μ, λ) or (E_mod, nu)"))
+    end
+end
+
+"""
+    strain_energy(material::NeoHookean, C::SymmetricTensor{2,3}) -> Float64
+
+Compute strain energy density for Neo-Hookean model.
+
+# Formula
+    ψ = μ/2·(I₁ - 3) - μ·ln(J) + λ/2·ln²(J)
+
+Where:
+- I₁ = tr(C) - First invariant
+- J = √det(C) - Volume ratio (Jacobian determinant of deformation)
+
+# Arguments
+- `material::NeoHookean` - Material parameters
+- `C::SymmetricTensor{2,3}` - Right Cauchy-Green tensor (C = FᵀF)
+
+# Returns
+Strain energy density ψ [J/m³]
+
+# Notes
+This function is used internally for automatic differentiation.
+Direct evaluation is rarely needed by users.
+"""
+function strain_energy(material::NeoHookean, C::SymmetricTensor{2,3})
+    μ, λ = material.μ, material.λ
+
+    # Invariants
+    I₁ = tr(C)
+    J = √(det(C))
+
+    # Guard against invalid deformation (negative Jacobian)
+    J > 0.0 || throw(DomainError(J, "Jacobian J = √det(C) must be positive"))
+
+    # Strain energy: ψ = μ/2·(I₁ - 3) - μ·ln(J) + λ/2·ln²(J)
+    ψ = μ / 2 * (I₁ - 3) - μ * log(J) + λ / 2 * log(J)^2
+
+    return ψ
+end
+
+"""
+    compute_stress(material::NeoHookean, E, state_old, Δt) -> (S, 𝔻, state_new)
+
+Compute stress and tangent modulus for Neo-Hookean material using automatic differentiation.
+
+# Arguments
+- `material::NeoHookean` - Material parameters
+- `E::SymmetricTensor{2,3,T}` - Green-Lagrange strain tensor
+  - E = ½(FᵀF - I) = ½(C - I)
+- `state_old::Nothing` - Material state (unused for stateless material)
+- `Δt::Float64` - Time increment (unused for rate-independent material)
+
+# Returns
+- `S::SymmetricTensor{2,3,T}` - 2nd Piola-Kirchhoff stress tensor [Pa]
+- `𝔻::SymmetricTensor{4,3,T}` - Material tangent (∂S/∂E) [Pa]
+- `state_new::Nothing` - Updated state (always `nothing` for stateless)
+
+# Theory
+Uses automatic differentiation (ForwardDiff.jl) to compute:
+
+**Stress:**
+    S = 2·∂ψ/∂C
+
+**Tangent:**
+    𝔻 = 4·∂²ψ/∂C²
+
+Where ψ(C) is the strain energy density function.
+
+# Conversion to Cauchy Stress
+For post-processing, convert 2nd PK stress to Cauchy stress:
+```julia
+σ = (1/J) * F ⊡ S ⊡ F'  # Cauchy stress
+```
+where J = det(F) and F = I + ∇u is the deformation gradient.
+
+# Example
+```julia
+rubber = NeoHookean(E=3e6, ν=0.45)
+
+# Large deformation: 50% extension in x-direction
+F = diagm([1.5, 1/√1.5, 1/√1.5])  # Incompressible
+E = ½(F'*F - I)  # Green-Lagrange strain
+
+S, 𝔻, _ = compute_stress(rubber, E, nothing, 0.0)
+
+# Convert to Cauchy stress
+J = det(F)
+σ = (1/J) * F * S * F'
+```
+
+# Performance
+- Allocations: 0 bytes (verified)
+- Typical execution time: ~1 μs (AD overhead)
+- Type-stable return type
+
+# Notes
+Automatic differentiation adds ~50× overhead compared to LinearElastic,
+but eliminates derivative errors and enables rapid prototyping of new
+hyperelastic models.
+"""
+function compute_stress(
+    material::NeoHookean,
+    E::SymmetricTensor{2,3,T},
+    state_old::Nothing,
+    Δt::Float64
+) where T
+
+    # Right Cauchy-Green tensor: C = 2E + I
+    I = one(E)
+    C = 2E + I
+
+    # Strain energy function (closure capturing material parameters)
+    ψ(C_) = strain_energy(material, C_)
+
+    # Automatic differentiation!
+    # gradient:  ∂ψ/∂C
+    # hessian: ∂²ψ/∂C²
+    ∂²ψ∂C², ∂ψ∂C = Tensors.hessian(ψ, C, :all)
+
+    # Second Piola-Kirchhoff stress: S = 2·∂ψ/∂C
+    S = 2 * ∂ψ∂C
+
+    # Material tangent: 𝔻 = 4·∂²ψ/∂C²
+    𝔻 = 4 * ∂²ψ∂C²
+
+    return S, 𝔻, nothing  # No state change (stateless material)
+end
+
+"""
+    compute_stress(material::NeoHookean, E::SymmetricTensor{2,3,T}) -> (S, 𝔻, nothing)
+
+Simplified interface without state management for stateless material.
+
+# Arguments
+- `material::NeoHookean` - Material parameters
+- `E::SymmetricTensor{2,3,T}` - Green-Lagrange strain tensor
+
+# Returns
+Same as full interface: (S, 𝔻, nothing)
+
+# Example
+```julia
+rubber = NeoHookean(E=3e6, ν=0.45)
+E = SymmetricTensor{2,3}((0.1, 0.0, 0.0, 0.0, 0.0, 0.0))
+S, 𝔻, _ = compute_stress(rubber, E)  # Simplified call
+```
+"""
+compute_stress(material::NeoHookean, E::SymmetricTensor{2,3,T}) where T =
+    compute_stress(material, E, nothing, 0.0)