Type | Intent | Optional | Attributes | Name | ||
---|---|---|---|---|---|---|

integer | :: | nI(NEl) |
||||

integer | :: | exflag |
||||

integer | :: | nSingleExcits |
||||

integer | :: | nDoubleExcits |

```
SUBROUTINE CountExcitations3(nI, exflag, nSingleExcits, nDoubleExcits)
! This routine simply counts the excitations in terms of single and doubles from the nI determinant.
! The exflag sent through indicates which should be counted - exflag=1 means only singles, exflag=2 means
! only doubles, and anything else both are counted.
USE SymData, only: nSymLabels
USE SystemData, only: ElecPairs, tFixLz, iMaxLz
USE GenRandSymExcitNUMod, only: PickElecPair, construct_class_counts, ClassCountInd, ScratchSize
INTEGER :: nSingleExcits, nDoubleExcits, Symi, i, Spini, nI(NEl)
INTEGER :: iSpn, Elec1Ind, Elec2Ind, SymProduct, exflag
INTEGER :: Syma, Symb, Spina, Spinb, StartSpin, EndSpin
INTEGER :: ClassCount2(ScratchSize), SumMl
INTEGER :: ClassCountUnocc2(ScratchSize)
INTEGER :: StartMl, EndMl, Mla, Mlb
CALL construct_class_counts(nI, ClassCount2, ClassCountUnocc2)
! This sets up arrays containing the number of occupied and unoccupied in each symmetry.
! ClassCounts2(1,:)=No alpha occupied, ClassCounts2(2,:)=No Beta occupied.
! ClassCountsUnocc2(1,:)=No alpha unocc, ClassCounts2Unocc2(2,:)=No Beta unocc.
! The second index of these arrays referrs to the symmetry (0 -> 7).
! Only counting. Run through each occupied orbital, and count the number of spin and symmetry allowed orbitals it
! may be excited to.
nSingleExcits = 0
nDoubleExcits = 0
IF (exflag /= 2) THEN
! Count the singles.
! Take each electron and find out the number of symmetry allowed orbitals it may be excited to.
do i = 1, NEl
Symi = SpinOrbSymLabel(nI(i))
IF ((G1(nI(i))%Ms) == -1) Spini = 2 ! G1(i)%Ms is -1 for beta, and 1 for alpha.
IF ((G1(nI(i))%Ms) == 1) Spini = 1 ! Translate this into 1 for alpha and 2 for beta
! for the ClassCount arrays.
IF (tFixLz) THEN
Mla = G1(nI(i))%Ml
ELSE
Mla = 0
end if
! This electron in orbital of SymI and SpinI can only be excited to orbitals with the same spin and symmetry.
! Then add in the number of unoccupied orbitals with the same spin and symmetry to which each electron may be excited.
nSingleExcits = nSingleExcits + ClassCountUnocc2(ClassCountInd(Spini, Symi, Mla))
end do
end if
! This is the end of the singles.
! write(stdout,*) 'Number of singles',nSingleExcits
! For the doubles, first pick an electron pair i,j.
! Based on these orbitals, run through each spin and each symmetry - take this to be orbital a.
! Multiply the number with these symmetries by the number of possible b orbitals which correspond.
! Do this for all a and then all i,j pairs.
IF (exflag /= 1) THEN
do i = 1, ElecPairs
! iSpn=2 for alpha beta pair, ispn=3 for alpha alpha pair and ispn=1 for beta beta pair.
CALL PickElecPair(nI, Elec1Ind, Elec2Ind, SymProduct, iSpn, SumMl, i)
StartSpin = 1
EndSpin = 2
IF (iSpn == 3) EndSpin = 1
IF (iSpn == 1) StartSpin = 2
do Spina = StartSpin, EndSpin ! Run through both spins, orbital a may be alpha or beta.
IF (iSpn == 2) THEN
! Spin of orbital b should be opposite to orbital a.
IF (Spina == 1) Spinb = 2
IF (Spina == 2) Spinb = 1
ELSE
! Spin of orbital b should be the same as orbital a.
IF (Spina == 1) Spinb = 1
IF (Spina == 2) Spinb = 2
end if
do Syma = 0, nSymLabels - 1
! Need to work out the symmetry of b, given the symmetry of a (Sym).
Symb = IEOR(Syma, SymProduct)
IF (tFixLz) THEN
StartMl = -iMaxLz
EndMl = iMaxLz
ELSE
StartMl = 0
EndMl = 0
end if
do Mla = StartMl, EndMl
Mlb = SumMl - Mla !Will be 0 if no Lz, otherwise we need Mla + Mlb = Mli + Mlj = SumMl
IF (ABS(Mlb) <= iMaxLz) THEN
IF ((Spina == Spinb) .and. (Syma == Symb) .and. (Mla == Mlb)) THEN
! If the spin and spatial symmetries of a and b are the same
! there will exist a case where Orba = Orbb, want to remove this.
nDoubleExcits = nDoubleExcits + (ClassCountUnocc2(ClassCountInd(Spina, Syma, Mla)) &
* (ClassCountUnocc2(ClassCountInd(Spinb, Symb, Mlb)) - 1))
ELSE
nDoubleExcits = nDoubleExcits + (ClassCountUnocc2(ClassCountInd(Spina, Syma, Mla)) &
* ClassCountUnocc2(ClassCountInd(Spinb, Symb, Mlb)))
end if
end if
end do
end do
end do
end do
nDoubleExcits = nDoubleExcits / 2
end if
ENDSUBROUTINE CountExcitations3
```

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