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I am trying to calculate the energies of the S1 and T1 states of a molecule with UDFT using Gaussian.

For the T1 state, I can successfully use the following input file:

#p opt freq ub3lyp/6-311+g* geom=checkpoint guess=mix nosymm

molecule1

0 3

For the S1 state, I tried:

#p opt freq ub3lyp/6-311+g* geom=checkpoint guess=mix nosymm

molecule1

1 1

but I received the following error:

"The combination of multiplicity 1 and 219 electrons is impossible."

What am I doing wrong?

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1 Answer 1

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"The combination of multiplicity 1 and 219 electrons is impossible."

219 is an odd number of electrons.

A spin multiplicity of 1 means that there are no unpaired electrons. This means that all electrons are paired, meaning that the number of electrons must be an even number. Therefore 219 electrons (or any odd number of electrons) is not compatible with having a spin multiplicity of 1.

Spin multiplicity is 2S+1, in which S is the spin. For the spin multplicity to be 1, you need S to be 0, which means that there's no unpaired electrons, because each unpaired electron would contribute a value of 1/2 to the spin (5 unpaired electrons would mean we have a spin of 5/2 and a spin multiplicity of 6).

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  • $\begingroup$ Thanks Nike, I understand your reply. However, to use '0 1' would mean to calculate the energy of the ground state S0. How can I calculate the S1? $\endgroup$
    – Laura
    Feb 23 at 14:52
  • $\begingroup$ @Laura that is a different question, for which I request that you make a new post, please. $\endgroup$ Feb 23 at 14:54
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    $\begingroup$ @Laura The simplest way is to do a TDDFT calculation. Please check the Gaussian manual: gaussian.com/td $\endgroup$
    – wzkchem5
    Feb 24 at 14:03
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    $\begingroup$ @Laura Note that that first number in 0 1 (or 0 3) is the charge; your input file (attempts to) calculate the singlet state of the molecule's cation, rather than an excited state of the neutral molecule. $\endgroup$
    – elutionary
    Feb 26 at 16:11

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