# Obtaining energy level estimates

Estimating the values of energy levels is one of the principal applications of quantum chemistry. Here, we outline how this may be performed for the canonical example of molecular Hydrogen. The sample referenced in this section is MolecularHydrogen in the chemistry samples repository. A more visual example that plots the output is the MolecularHydrogenGUI demo.

Our first step is to construct the Hamiltonian representing molecular Hydrogen. Though this can be constructed through the NWChem tool, we manually add Hamiltonian terms for brevity in this sample.

    // These orbital integrals are represented using the OrbitalIntegral
// data structure.
var energyOffset = 0.713776188; // This is the coulomb repulsion
var nElectrons = 2; // Molecular hydrogen has two electrons
var orbitalIntegrals = new OrbitalIntegral[]
{
new OrbitalIntegral(new[] { 0,0 }, -1.252477495),
new OrbitalIntegral(new[] { 1,1 }, -0.475934275),
new OrbitalIntegral(new[] { 0,0,0,0 }, 0.674493166),
new OrbitalIntegral(new[] { 0,1,0,1 }, 0.181287518),
new OrbitalIntegral(new[] { 0,1,1,0 }, 0.663472101),
new OrbitalIntegral(new[] { 1,1,1,1 }, 0.697398010),
// This line adds the identity term.
new OrbitalIntegral(new int[] { }, energyOffset)
};

// We initialize a fermion Hamiltonian data structure and add terms to it.
var fermionHamiltonian = new OrbitalIntegralHamiltonian(orbitalIntegrals).ToFermionHamiltonian();


Simulating the Hamiltonian requires us to convert the fermion operators to qubit operators. This conversion is performed through the Jordan-Wigner encoding as follows.

    // The Jordan-Wigner encoding converts the fermion Hamiltonian,
// expressed in terms of fermionic operators, to a qubit Hamiltonian,
// expressed in terms of Pauli matrices. This is an essential step
// for simulating our constructed Hamiltonians on a qubit quantum
// computer.
var jordanWignerEncoding = fermionHamiltonian.ToPauliHamiltonian(Pauli.QubitEncoding.JordanWigner);

// We also need to create an input quantum state to this Hamiltonian.
// Let us use the Hartree-Fock state.
var fermionWavefunction = fermionHamiltonian.CreateHartreeFockState(nElectrons);

// This Jordan-Wigner data structure also contains a representation
// of the Hamiltonian and wavefunction made for consumption by the Q# operations.
var qSharpHamiltonianData = jordanWignerEncoding.ToQSharpFormat();
var qSharpWavefunctionData = fermionWavefunction.ToQSharpFormat();
var qSharpData = QSharpFormat.Convert.ToQSharpFormat(qSharpHamiltonianData, qSharpWavefunctionData);


We now pass the qSharpData representing the Hamiltonian to the TrotterStepOracle function in Simulating Hamiltonian dynamics. TrotterStepOracle returns a quantum operation that approximates the real time-evolution of the Hamiltonian.

// qSharpData passed from driver
let qSharpData = ...

// Choose the integrator step size
let stepSize = 1.0;

// Choose the order of the Trotter—Suzuki integrator.
let integratorOrder = 4;

// oracle is an operation that applies a single time-step of evolution for duration stepSize.
// rescale is just 1.0/stepSize -- the number of steps required to simulate unit-time evolution.
// nQubits is the number of qubits that must be allocated to run the oracle operatrion.
let (nQubits, (rescale, oracle)) =  TrotterStepOracle (qSharpData, stepSize, integratorOrder);


We can now use the standard library's phase estimation algorithms to learn the ground state energy using the above simulation. This requires preparing a good approximation to the quantum ground state. Suggestions for such approximations are provided in the Broombridge schema, but absent these suggestions, the default approach adds a number of hamiltonian.NElectrons electrons to greedily minimize the diagonal one-electron term energies. The phase estimation functions and operations are located in the [Microsoft.Quantum.Characterization namespace](xref:microsoft.quantum.characterization in DocFX notation).

The following snippet shows how the real time-evolution output by the chemistry simulation library may be integrated with quantum phase estimation.

operation GetEnergyByTrotterization (
qSharpData : JordanWignerEncodingData,
nBitsPrecision : Int,
trotterStepSize : Double,
trotterOrder : Int) : (Double, Double) {

// The data describing the Hamiltonian for all these steps is contained in
// qSharpData
let (nSpinOrbitals, fermionTermData, statePrepData, energyOffset) = qSharpData!;

// We use a Product formula, also known as Trotterization to
// simulate the Hamiltonian.
let (nQubits, (rescaleFactor, oracle)) =
TrotterStepOracle(qSharpData, trotterStepSize, trotterOrder);

// The operation that creates the trial state is defined below.
// By default, greedy filling of spin-orbitals is used.
let statePrep = PrepareTrialState(statePrepData, _);

// We use the Robust Phase Estimation algorithm
// of Kimmel, Low and Yoder.
let phaseEstAlgorithm = RobustPhaseEstimation(nBitsPrecision, _, _);

// This runs the quantum algorithm and returns a phase estimate.
let estPhase = EstimateEnergy(nQubits, statePrep, oracle, phaseEstAlgorithm);

// We obtain the energy estimate by rescaling the phase estimate
// with the trotterStepSize. We also add the constant energy offset
// to the estimated energy.
let estEnergy = estPhase * rescaleFactor + energyOffset;

// We return both the estimated phase, and the estimated energy.
return (estPhase, estEnergy);
}


This Q# code may now be invoke from the driver program. In the following, we create a full-state simulator and run GetEnergyByTrotterization to obtain the ground state energy.

using (var qsim = new QuantumSimulator())
{
// We specify the bits of precision desired in the phase estimation
// algorithm
var bits = 7;

// We specify the step-size of the simulated time-evolution. This needs to
// be small enough to avoid aliasing of phases, and also to control the
// error of simulation.
var trotterStep = 0.4;

// Choose the Trotter integrator order
Int64 trotterOrder = 1;

// As the quantum algorithm is probabilistic, let us run a few trials.

// This may be compared to true value of
Console.WriteLine("Exact molecular Hydrogen ground state energy: -1.137260278.\n");
Console.WriteLine("----- Performing quantum energy estimation by Trotter simulation algorithm");
for (int i = 0; i < 5; i++)
{
// EstimateEnergyByTrotterization
var (phaseEst, energyEst) = GetEnergyByTrotterization.Run(qsim, qSharpData, bits, trotterStep, trotterOrder).Result;
}
}


Note that two parameters are returned. energyEst is the estimate of the ground state energy, and should be around -1.137 on average. phaseEst is the raw phase returned by the phase estimation algorithm, and is useful to diagnose when aliasing occurs due to a trotterStep that is too large.