Create an Orbital Optimizer

Goal

Build and configure an orbital optimizer using the fluent builder API. Choose between Newton and simple optimizers. Optionally, set estimators, minimizers, gradient calculators, and number of shots for sampling.

Prerequisites

A basic understanding of orbital optimization (rotating molecular orbitals to lower the system’s energy)

Overview

Start the builder with:

import qrunch as qc

opt = qc.orbital_optimizer_creator()

Then narrow to a specific algorithm and call .create():

optimizer = (
    qc.orbital_optimizer_creator()
    .newton()  # or .simple()
    # .<configure>(...)
    .create()
)

Available Optimizers

Newton optimizer (recommended)

A Newton-style optimizer that uses a gradient calculator. Supports finite-difference or local (RDM-based) gradients.

import qrunch as qc

optimizer = (
    qc.orbital_optimizer_creator()
    .newton()
    .choose_gradient_calculator()
      .local_gradient(                  # analytical (RDM-based) gradients
          estimator=qc.estimator_creator().excitation_gate().create()
      )
    .with_shots(None)                   # None = exact estimator for gradients
    .with_options(                      # optional Newton options
        options=qc.options.NewtonMinimizerOptions(...)
    )
    .create()
)

The Newton-style optimizer with a local gradient calculator is a strong default choice for orbital optimization. It leverages analytical gradients via reduced density matrices (RDMs), leading to efficient and accurate updates. Options can be set using a NewtonMinimizerOptions instance.

Simple optimizer

A lightweight optimizer that uses an estimator and a classical minimizer.

import qrunch as qc

optimizer = (
    qc.orbital_optimizer_creator()
    .simple()
    .with_estimator(                    # optional: set custom estimator
        estimator=qc.estimator_creator().excitation_gate().create()
    )
    .choose_minimizer()                 # optional: choose custom minimizer
      .scipy(
         name="L-BFGS-B",
         options=qc.options.ScipyMinimizerOptions(...)
      )
    .with_shots(None)                   # None = exact estimator; or an int for finite shots
    .create()
)

See Choose a Minimizer for a guide to choosing a minimizer.

Next Step

Use the orbital optimizer within VQE calculator variants that support orbital rotations (e.g., OO-BEAST-VQE, OO-FAST-VQE).