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I'm trying to compute bond orders in a molecule, let's consider S2 as an example. I'm not entirely sure what's the right way to do that, but I assume NBO (Natural Bond Orbital) should give me what I need. For calculations I'm using MOLPRO. I've tried to launch it with UCCSD(T)-F12 as follows:

 symmetry, nosym
 geometry = {
 S1
 S2, S1, bond
 }
 bond = 3.60529338 BOHR
 basis = cc-pVTZ-F12
 set, spin = 0

 hf
 uccsd(t)-f12a, scale_trip = 1
 nbo, keep_wbi = 1

It seemed to work and gave me the following output:

  - - - - - - - - - - - - - - - - - - - - - - - -
        NBO  OCCUPATION       LABEL     CENTERS
  - - - - - - - - - - - - - - - - - - - - - - - -
         1    2.000000         CR       1
         2    1.999827         CR       1
         3    1.999918         CR       1
         4    1.999989         CR       1
         5    1.999848         CR       1
         6    2.000000         CR       2
         7    1.999827         CR       2
         8    1.999918         CR       2
         9    1.999989         CR       2
        10    1.999848         CR       2
        11    1.999151         LP       1
        12    1.989845         LP       1
        13    1.999151         LP       2
        14    1.989845         LP       2
        15    2.000000         BD       1    2
        16    2.000000         BD       1    2

CR seems to stand for core, LP - lone pair, BD - bonding. From this I can conclude the bond order is 2, since we have two bonding orbitals with 2 electrons on each. However, the spin state was set to 0 here, whereas it's supposed to be 2 (2 unpaired electrons). When I tried to set spin to 2, the same calculation failed:

No suitable structure found!
 ? Error
 ? The problem occurs in nbo_transform

Can anyone suggest what is wrong here? Are there any other ways to compute bond order?

The full outputs: spin=0; spin=2

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  • 2
    $\begingroup$ You could try the many methods by Multiwfn for bond orders. $\endgroup$ – pH13 - Yet another Philipp Jun 12 '18 at 17:01

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