Spin of supermolecule in SAPT0

Hi,

I want to fragment ethane as two methyl radicals. This is what I tried:

# Geometry @ PBE/cc-pVDZ w/ ORCA 3
memory 4000 mb
set {
 basis jun-cc-pVDZ
 reference uhf
# scf_type df
}
molecule {
 0 2
  H   -0.97395637142895     -0.33919214271350     -0.41740669835522
  H   0.78074222854571     -0.67377559545544     -0.41707405986210
  H   0.19295516133507      1.01310450698540     -0.41737407148921
  C   0.00003917953494      0.00004478839706     -0.00814962304354
 --
 0 2
  C   -0.00003917886785     -0.00004479102906      1.52020761150977
  H   -0.78074222517214      0.67377559966970      1.92913204324353
  H   -0.19295516553330     -1.01310450794172      1.92943206399842
  H   0.97395637158652      0.33919214208754      1.92946469039835
 units angstrom
 symmetry c1
 no_reorient
 no_com
}
energy('sapt0')

Expecting a negative interaction energy, I found, to my surprise, the following:

    SAPT Results
  --------------------------------------------------------------------------------------------------------
    Electrostatics               -223.59373582 [mEh]    -140.30719337 [kcal/mol]    -587.04535340 [kJ/mol]
      Elst10,r                   -223.59373582 [mEh]    -140.30719337 [kcal/mol]    -587.04535340 [kJ/mol]

    Exchange                      679.89602607 [mEh]     426.64121537 [kcal/mol]    1785.06701643 [kJ/mol]
      Exch10                      679.89602607 [mEh]     426.64121537 [kcal/mol]    1785.06701643 [kJ/mol]
      Exch10(S^2)                 338.46393219 [mEh]     212.38933286 [kcal/mol]     888.63705396 [kJ/mol]

    Induction                    -223.53136555 [mEh]    -140.26805543 [kcal/mol]    -586.88160024 [kJ/mol]
      Ind20,r                    -424.32601598 [mEh]    -266.26860613 [kcal/mol]   -1114.06795496 [kJ/mol]
      Exch-Ind20,r                 28.86291334 [mEh]      18.11175232 [kcal/mol]      75.77957898 [kJ/mol]
      delta HF,r (2)              171.93173709 [mEh]     107.88879838 [kcal/mol]     451.40677574 [kJ/mol]

    Dispersion                    -48.51926896 [mEh]     -30.44630220 [kcal/mol]    -127.38734064 [kJ/mol]
      Disp20                      -51.54141750 [mEh]     -32.34272913 [kcal/mol]    -135.32199165 [kJ/mol]
      Exch-Disp20                   3.02214855 [mEh]       1.89642692 [kcal/mol]       7.93465101 [kJ/mol]

  Total HF                        232.77092470 [mEh]     146.06596657 [kcal/mol]     611.14006280 [kJ/mol]
  Total SAPT0                     184.25165574 [mEh]     115.61966437 [kcal/mol]     483.75272215 [kJ/mol]
  --------------------------------------------------------------------------------------------------------

A repulsive interaction. Can this be possibly correct? Well, I thought this could have something to do with spin:

$ grep @ etano.out
...
# supermolecule
   @Spin Contamination Metric:   8.131259654E-03
   @S^2 Expected:                7.500000000E-01
   @S^2 Observed:                7.581312597E-01
   @S   Expected:                5.000000000E-01
   @S   Observed:                5.000000000E-01
  @DF-UHF Final Energy:   -39.55040227009846

This is not the multiplicity I wanted. How can I select the multiplicity of the final molecule using SAPT0? Is it even possible?

Thanks in advance,
Felipe

SAPT can only produce the high spin result currently. The result are therefore typically replulsive.

Future research may allow the computation of all spin states simultaneously.

Thanks for your prompt response. I’ll be looking forward to that.

All the best,
Felipe