public static UnitaryCCWavefunction CreateAllUCCSDSingletExcitations()

in Chemistry/src/DataModel/Fermion/Wavefunction/WavefunctionExtensions.cs [22:75]
45 lines of code
11 McCabe index (conditional complexity)

        public static UnitaryCCWavefunction<SpinOrbital> CreateAllUCCSDSingletExcitations(this IEnumerable<SpinOrbital> occupiedSpinOrbitals, int nOrbitals)
        {
            // Create list of unoccupied spin-orbitals
            var virtualOrbitals = Enumerable.Range(0, nOrbitals).Select(x => new SpinOrbital(x, Spin.u))
                .Concat(Enumerable.Range(0, nOrbitals).Select(x => new SpinOrbital(x, Spin.d)))
                .Except(occupiedSpinOrbitals);

            var outputState = new UnitaryCCWavefunction<SpinOrbital>()
            {
                Reference = new SingleCFWavefunction<SpinOrbital>(occupiedSpinOrbitals)
            };

            // Populate singles excitations.
            foreach (var occupiedIdx in occupiedSpinOrbitals)
            {
                foreach (var virtualIdx in virtualOrbitals)
                {
                    if (occupiedIdx.Spin == virtualIdx.Spin)
                    {
                        var term = new IndexOrderedSequence<SpinOrbital>(new[] { virtualIdx, occupiedIdx }.ToLadderSequence());
                        outputState.Set(term, new Complex());
                    }
                }
            }

            // Populate doubles excitations.
            foreach (var occupiedIdx0 in occupiedSpinOrbitals)
            {
                foreach (var occupiedIdx1 in occupiedSpinOrbitals)
                {
                    // Only loop over distinct pairs of occupied spin orbitals
                    if (occupiedIdx0.ToInt() < occupiedIdx1.ToInt())
                    {
                        foreach (var virtualIdx0 in virtualOrbitals)
                        {
                            foreach (var virtualIdx1 in virtualOrbitals)
                            {
                                // Only loop over distinct pairs of occupied spin orbitals
                                if (virtualIdx0.ToInt() < virtualIdx1.ToInt())
                                {
                                    if (virtualIdx0.Spin + virtualIdx1.Spin == occupiedIdx0.Spin + occupiedIdx1.Spin)
                                    {
                                        var term = new IndexOrderedSequence<SpinOrbital>(new[] { virtualIdx1, virtualIdx0, occupiedIdx1, occupiedIdx0 }.ToLadderSequence());
                                        outputState.Set(term, new Complex());
                                    }
                                }
                            }
                        }
                    }
                }
            }
            return outputState;

        }