number of electrons report

[mephisto3142001]

I calculated the Molecular Energy Spectrum of DTB and saw number of electrons report . I thought that the number of valence electrons is 40(6x4(Carbon)+1x4(Hydrogen)+6x2(Sulfur)=40). Furthermore, there is a statement "An isolate DTB molecule has 40 valence electrons" in paper, Comp. Mat. Sci., 27, 151, 2003.
However, the result I got showed  Number of electrons = 42.000000. Please help me solve this problem.

+------------------------------------------------------------------------------+
| Molecular Energy Spectrum Report                                             |
| ---------------------------------------------------------------------------- |
| Fermi level = -2.452011e+00                                                  |
| Number of electrons = 42.000000                                              |
| Unit = eV                                                                    |
| Eigenenergies given relative to the Fermi Level                              |
+------------------------------------------------------------------------------+

    0  -1.762873e+01   2.000000e+00
    1  -1.610272e+01   2.000000e+00
    2  -1.459204e+01   2.000000e+00
    3  -1.450402e+01   2.000000e+00
    4  -1.276223e+01   2.000000e+00
    5  -1.121167e+01   2.000000e+00
    6  -1.042559e+01   2.000000e+00
    7  -8.782769e+00   2.000000e+00
    8  -7.705019e+00   2.000000e+00
    9  -7.369467e+00   2.000000e+00
   10  -6.677567e+00   2.000000e+00
   11  -6.438039e+00   2.000000e+00
   12  -5.800967e+00   2.000000e+00
   13  -5.079045e+00   2.000000e+00
   14  -4.709258e+00   2.000000e+00
   15  -4.549146e+00   2.000000e+00
   16  -3.290120e+00   2.000000e+00
   17  -3.281403e+00   2.000000e+00
   18  -3.142225e+00   2.000000e+00
   19  -2.588342e+00   2.000000e+00
   20  -1.804314e+00   2.000000e+00
   21   2.410416e+00   6.424856e-41

[Anders Blom]

Was this for a pure DTB molecule or a projection on the molecule sitting in the device configuration? If you post the input script for the calculation it might be easier to answer.

[mephisto3142001]

It was the projection on the DTB molecule.
Why does the projection have two additional electrons than the isolated DTB?

[Anders Blom]

Another guess is that you have di-thiolate-benzene rather than di-tiole-benzene.

But, as mentioned often on this Forum, unless we can see your input script, guessing is the best we can do, and that is - honestly speaking - often a waste of everyone's time.

[mephisto3142001]

here is my script.
please take a look

[Anders Blom]

The isolated benzene-dithiole molecule actually has 42 electrons, however in "Au-DTB" system you work with benzene-dithiolate (or if you prefer to call it dithiolate-benzene, DTB), which has 40. In this case the S atoms have each a dangling bond, so it's natural to have charge transfer of one electron from the metal to the benzene-dithiolate in the junction configuration. The case with thiol bonds to the surface would also give 42 electrons, but in this case there are 2 extra explicit H atoms. You can see both these systems in the article you quoted.