/// <summary>
/// Enumerates over all spin-orbital integrals represented
/// by this term.
/// These symmetries arise from:
/// - Indistinguishability of electrons.
/// - Orbitals are assumed to be real.
/// - electron spins must be paired.
/// </summary>
/// <returns>
/// Array of spin-orbital orbital integrals with the same coefficient.
/// </returns>
public SpinOrbital[][] EnumerateSpinOrbitals()
{
// Assumes spinOrbitals has an even number of elements
// Only index over like spins S1 S2 S3 ... S3 S2 S1
const Int64 nSpins = 2L;
var nSpinOrbitalArrays = nSpins.Pow(OrbitalIndices.Length / 2);
SpinOrbital[][] spinOrbitalArrayOfArray = new SpinOrbital[OrbitalIndices.Length][];
for (int idx = 0; idx < nSpinOrbitalArrays; idx++)
{
SpinOrbital[] spinOrbitalArray = new SpinOrbital[OrbitalIndices.Length];
for (int idxOrbital = 0; idxOrbital < OrbitalIndices.Length / 2; idxOrbital++)
{
var fst = idxOrbital;
var lst = OrbitalIndices.Length - idxOrbital - 1;
var spin = (idx / nSpins.Pow(idxOrbital)) % nSpins;
spinOrbitalArray[fst] = new SpinOrbital
{
orbital = OrbitalIndices[fst],
spin = spin
};
spinOrbitalArray[lst] = new SpinOrbital
{
orbital = OrbitalIndices[lst],
spin = spin
};
}
spinOrbitalArrayOfArray[idx] = spinOrbitalArray;
}
return(spinOrbitalArrayOfArray);
}
/// <summary>
/// FermionTerm constructor.
/// </summary>
public FermionTerm(Int64 nOrbitals, Int64[] caArray, Int64[] fermionIdxArray, Double coeffIn)
{
CreationAnnihilationIndices = caArray;
SpinOrbitalIndices = SpinOrbital.ToSpinOrbitals(nOrbitals, fermionIdxArray);
coeff = coeffIn;
if (!IsValid())
{
throw new System.ArgumentException(
$"Invalid FermionTerm specified. Length of caArray {caArray} and soArray {fermionIdxArray} must be equal.",
"caArray"
);
}
}
