Homolytic dissociation of singlet lithium dimer
Clicked A Few Times
4:38:42 AM PDT - Sat, Apr 26th 2014
Hi, all.

I'm doing calculation of a dissociation curve of singlet lithium dimer using UHF module.

The point is that the spin multiplicity would be staying at singlet state so that one alpha electron is localized on one lithium atom, one beta electron on the other.

However, when I did a simple UHF calculation without any additional directives, the wavefunction remained symmetric for both alpha and beta electrons until dissociation limit.

So I divided wavefunction into fragments to make a guess orbital that is localized on each atom.

Here is my input.

geometry lidimer noautosym units angstrom
    Li                        8.00     0.00    0.00

    Li                        0.00     0.00    0.00
END

geometry li1 noautosym units angstrom
    Li                        8.00     0.00    0.00
END

geometry li2 noautosym units angstrom
    Li                        0.00     0.00    0.00
END

BASIS spherical
 Li   library  def2-qzvpp
end

SET geometry li1
SCF
 DOUBLET

 UHF

 ADAPT OFF

 SYM OFF

 VECTORS INPUT atomic OUTPUT li1.movecs
END
TASK SCF

SET geometry li2
SCF
 DOUBLET

 UHF

 ADAPT OFF

 SYM OFF

 VECTORS INPUT atomic OUTPUT li2.movecs
END
TASK SCF

SET geometry lidimer
SCF
 UHF

 SINGLET

 ADAPT OFF

 SYM OFF

 VECTORS INPUT fragment li1.movecs li2.movecs OUTPUT lidimer.movecs
END
task scf

However, this calculation causes an error that spin multiplicity is not correct in the dimer.

Here's my error message.

------------------------------------------------------------------------

movecs_fragment: open shell mismatch        3

------------------------------------------------------------------------

I think the program innately assigned the sum of doublet should always be triplet, but I think it should be corrected because there are counterexamples like my calculation..

Do you have any idea to do this calculation successfully?

Thanks in advance.

Forum Vet
10:52:33 AM PDT - Mon, Apr 28th 2014
Kmh603
You can easily get your desired open-shell singlet solution by using the DFT module (and choosing
HF exchange), by selecting a multiplicity of -2 for the second lithium atom.
The input below shows all the needed steps.
start li2

geometry lidimer noautosym units angstrom
    Li                        8.00     0.00    0.00
    Li                        0.00     0.00    0.00
END

geometry li1 noautosym units angstrom
    Li                        8.00     0.00    0.00
END

geometry li2 noautosym units angstrom
    Li                        0.00     0.00    0.00
END

BASIS spherical
 Li   library  def2-qzvpp
end

SET geometry li1

dft
 xc hfexch
 mult 2
 ADAPT OFF
 SYM OFF
 VECTORS INPUT atomic OUTPUT li1.movecs
END
TASK dft

SET geometry li2
dft
 mult -2
 VECTORS INPUT atomic OUTPUT li2.movecs
END
TASK dft

SET geometry lidimer
dft
 odft
 mult 1
 VECTORS INPUT fragment li1.movecs li2.movecs OUTPUT lidimer.movecs
 mulliken
END
task dft


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