/// <summary>
/// Constructs a single-reference wave function. This
/// is treated as sparse multi-reference wave function with only
/// one term.
/// </summary>
/// <param name="term">
/// Sequence of indices of creation operators acting
/// on the vacuum state.
/// </param>
/// <example>
/// Create a list of indices of the creation operators, then
/// Convert the list of indices to a `FermionWavefunction` instance.
/// <code>
/// var indices = new[] { 1, 2, 6 };
/// var wavefunction = new FermionWavefunction<int>(indices);
/// </code>
/// </example>
public FermionWavefunction(IEnumerable <TIndex> term)
{
// This is deliberately set to SparseMultiConfigurational
// instead of SingleConfigurational as it is equivalent
// to a sparse multi-reference wavefunciton with only
// one term.
Method = StateType.SparseMultiConfigurational;
var singleReferenceWavefunction = new SingleCFWavefunction <TIndex>(term);
MCFData.Set(singleReferenceWavefunction, new Complex(1.0, 0.0));
}
/// <summary>
/// This approximates the Hamiltonian ground state by a greedy algorithm
/// that minimizes only the PP term energies. If there are no PP terms,
/// states will be occupied in lexicographic order.
/// </summary>
/// <returns>
/// Greedy trial state for minimizing Hamiltonian diagonal one-electron energy.
/// </returns>
public static FermionWavefunction <int> CreateHartreeFockState(this FermionHamiltonian hamiltonian, int nElectrons)
{
SingleCFWavefunction <int> greedyState = hamiltonian.GreedyStatePreparationSCF(nElectrons);
var wavefunction = new FermionWavefunction <int>
{
Energy = 0.0,
Method = StateType.SparseMultiConfigurational,
MCFData = new SparseMultiCFWavefunction <int>()
};
wavefunction.MCFData.Set(greedyState, new System.Numerics.Complex(1.0, 0.0));
return(wavefunction);
}
/// <summary>
/// Construct a copy of the input instance.
/// </summary>
/// <param name="term">Sequence of ladder operators.</param>
internal SingleCFWavefunction(SingleCFWavefunction <TIndex> term) : base(term)
{
}
/// <summary>
/// Constructor for empty instance.
/// </summary>
public SparseMultiCFWavefunction()
{
Reference = new SingleCFWavefunction <TIndex>();
Excitations = new Dictionary <IndexOrderedSequence <TIndex>, Complex>();
}