One question !

[Dibyendu Sardar]

Dear mopro users,

Recently, I have been looking at the optimized singlet and triplet surface of CaF-CaF.

I found the depth of the well for the global minimum of singlet surface is 18923 cm^-1. For the triplet surface, the depth of the well is 18074 cm^-1.

I found that the energy difference between these two surfaces for the  global minimum is only 849 cm^-1. We could not understand why the energy of the global minimum looks similar for the two different surfaces ? Are there any special reasons ?

We initially make some observations by analysing the molecular orbitals. We note that HOMO of the singlet optimized state is bonding and LUMO is anti-bonding characters. Again, for the triplet optimized geometry, the HOMO is an anti-bonding character.

The orbital coefficients for HOMO of triplet are

Orb     Occ        Energy       Coefficients

20.0    +            -0.1891       1-Ca  1s(0.628), Ca  2py(0.232);  Ca 2pz(-0.3011) ; 3-Ca 1s(-0.6283), 3-Ca 2py(-0.238), 3-Ca 2pz(-0.3001)

But, for the singlet state, orbital coefficient for HOMO are

19.1    2            -0.1407        1-Ca 1s(0.597),  1- Ca  2py(-0.1) , 1-Ca 2pz(0.125), 1-Ca 1s(0.181); 3-Ca 1s(0.5977), 3-Ca 2py(0.1), 3-Ca 2pz(0.125), 3-Ca 1s(0.181)

Therefore, in both cases the major contributions are coming from the 4s orbitals of two Ca atoms and minor contributions from 4p of Ca atoms. But, a very weak contribution from two F atoms.

But, how we could explain the small difference in energy for singlet and triplet optimized geometry by seeing the attached molecular orbitals figures.

Could you kindly assist me to explain such observations in the case of optimized geometry of singlet and triplet states of CaF-CaF?

For your reference. I am  attaching HOMO & LUMO of singlet optimized geometry, including LUMO of triplet.  Please let me know about your valuable responses in this regard.

Regards

Dr Dibyendu Sardar

JILA, University of Colorado Boulder, USA

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Dibyendu