Singlet carbene geometry opt

Hi there, I was just reproducing a result of Halogen Bonding with Carbene Bases by Janet E. Del Bene, Ibon Alkorta, José Elguero. I tried to manipulate the XB donor by using F2 and FCl. What I am getting for F2 geometry using df-mp2/aug-cc-pvtz (ri) is this geometry for F2, which seems logical.

Geometry for iteration 54
C 0.7355091409 -0.2507819383 0.0837757984
O 2.0251665222 -0.5243169315 0.1558567173
H 2.1455192658 -1.4064822965 0.5345176268
O 0.2582679872 0.7560527988 -0.3361518743
H -1.4966846180 1.1046656266 -0.4279404948
F -0.0032563947 -1.2662485399 0.5517135660
F -2.4184355426 1.2447426136 -0.4586547155

but for FCl, it is not, because the F-Cl bond distance is 1.9A, and Cl…C is 1.7A. this should not be logical. Is there any mistake?
The XYZ file is provided here.
Geometry for iteration 34
C 1.0550131920 0.0843167491 -0.0135345237
O 1.8671216474 -0.9127540537 0.1683663338
H 1.3291580881 -1.7107324989 0.3093697087
O 1.6936867384 1.1977781518 -0.2108583486
H 1.0324915595 1.8993917251 -0.3393343583
CL -0.6498508610 -0.0529256710 0.0045232815
F -2.5933851395 -0.2070206857 0.0331269971

Can you clarify what you mean? You are doing FCI optimizations of such a large system?

I am trying to optimize the halogen bonded (XB) complex between FCl (sigma-hole on Cl) and (OH)2C (singlet carbene).
The paper I mentioned have used different XB donor molecule, which I changed to F2 and FCl. I have found satisfactory result with F2, but not with FCl.

ah ok. I cannot say much about the chemical problem.
The system is small enough to allow CCSD(T) optimizations. Perhaps worth a try.
I’d recommend the DF-CCSD(T) code: set cc_type df

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@hokru, DF-CCSD(T) is taking to long.
I was wondering, If I can use mixed basis set, (non augmented ) for H. Since, H is not used for halogen bonding. In the paper I mentioned, they also have deducted f- functions from H atom.
If that is possible, how can I do that, I am trying options like this,

basis {
assign H cc-pvtz
assign aug-cc-pvtz
df_basis_mp2 H cc-pvtz-ri
df_basis_mp2 aug-cc-pvtz-ri

shows an error,
Conflicting basis set specification: assign lines present but shells have no [basname] label

I am trying with,
basis {
assign H cc-pvtz
assign aug-cc-pvtz
df_basis_mp2 {
assign H cc-pvtz-ri
assign aug-cc-pvtz-ri

Lets see if that works

It works for some systems, but not for all; such as FCl…CH2. I don’t know what to do. I have mentioned C2v symmetry which is followed through the optimization. the optimized structure should maintain C2v symmetry too. in this regard the <FClC is 180d, but the bond length is wrong. It shows r(F-Cl)=1.9A, while r(C-Cl)=1.6A.
Probably scanning the bond length along F-Cl…C might help. But it seems tough, trying to do so