---
myst:
  html_meta:
    description: Second-order optimizers in PyTorch C++ — LBFGS optimizer for full-batch optimization.
    keywords: PyTorch, C++, LBFGS, second-order, optimizer, full-batch
---

# Second-Order Optimizers

Second-order methods use curvature information (Hessian or its approximations)
to make better optimization steps. They can converge faster but are more
computationally expensive and memory-intensive.

## LBFGS (Limited-memory Broyden-Fletcher-Goldfarb-Shanno)

LBFGS is a quasi-Newton method that approximates the inverse Hessian using
gradient history. It can converge much faster than first-order methods for
smooth, convex-like loss surfaces.

**When to use:**

- Small models where memory isn't a concern
- Fine-tuning pre-trained models
- Convex or near-convex optimization problems
- Full-batch training (not mini-batch)

**Key parameters:**

- `lr`: Learning rate (often 1.0 for LBFGS)
- `max_iter`: Maximum iterations per step
- `history_size`: Number of past gradients to store

**Important:** LBFGS requires a closure function that recomputes the loss.

```{doxygenclass} torch::optim::LBFGS
:members:
:undoc-members:
```

**Example:**

```cpp
auto optimizer = torch::optim::LBFGS(
    model->parameters(),
    torch::optim::LBFGSOptions(1.0)
        .max_iter(20)
        .history_size(10));

// LBFGS requires a closure that recomputes the model
for (int epoch = 0; epoch < num_epochs; ++epoch) {
    auto closure = [&]() {
        optimizer.zero_grad();
        auto output = model->forward(data);
        auto loss = loss_fn(output, target);
        loss.backward();
        return loss;
    };
    optimizer.step(closure);
}
```
