orbitallabels = {"1s1/2","2s1/2","2p1/2","2p3/2"}
ind, NF = CreateAtomicIndicesList(orbitallabels,{{"Kappas",true}})
G, F = ReadFPLOBasisFunctions(orbitallabels,"+fval.001.1", "+fval.001.2")

U = NewOperator("AtomicU",NF,ind,G,F) -- creates full Coulomb interaction

-- now occupation of the s-orbitals shall be conserved

function Take(list,n1,n2)
  local result={}
  if n2== nil then
    if n1 > 0 then
      for i = 1, n1 do
        result[i]=list[i]
      end
    else
      for i=1,-n1 do
        result[i]=list[#list+n1+i]
      end
    end
   else
     for i = n1, n2 do
       result[i+1-n1] = list[i]
     end
   end
  return result
end

n0 = 3
n1 = 4

indConserved = Take(ind,1,n0-1)
indConserved.kappas = Take(ind.kappas,1,n0-1)
GConserved = Take(G,1,n0-1)
FConserved = Take(F,1,n0-1)

indNonCnsrvd = Take(ind,n0,n1)
indNonCnsrvd.kappas = Take(ind.kappas,n0,n1)
GNonCnsrvd = Take(G,n0,n1)
FNonCnsrvd = Take(F,n0,n1)

U0 = NewOperator("AU",NF,indConserved,GConserved,FConserved,{{"conserve",true}}) -- Coulomb interaction between conserved orbitals only

U1 = NewOperator("AU",NF,ind,G,F,indConserved.kappas,indNonCnsrvd.kappas,{{"conserve",true},{"1P-Operator",true}}) -- Coulomb interaction between conserved orbitals and others transformed to a one-particle operator given that the conserved orbitals are always completely filled.

U2 = NewOperator("AU",NF,indNonCnsrvd,GNonCnsrvd,FNonCnsrvd) -- Coulomb interaction between non-conserved orbitals

Uconserved = U0 + U1 + U2