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I'm making a topological analysis of a nanotube interacting with some metals.

The steps for the calculations were:

  • Optimizing the geometry (I am using SIESTA code).
  • Obtain the electronic density for the final relaxed structure (in cube format with the help of DENCHAR utility that is part of the SIESTA distribution): density files for total electron density (RHO), spin up electron density (RHO.UP) and spin down electron density (RHO.DOWN).
  • Run topological analysis with CRITIC21,2 software.

For the three metals under study, I got a bond critical point from the RHO file.
When analyzing the RHO.UP and RHO.DOWN files, for two metals I got two critical points (one for each spin). But for one metal, I only got one bond critical point.

As the recommended way to run topological studies is using the wavefunction directly instead the electron density, I run the topological analysis from density files with different grid size.

The question: Can I have a bond formed for only one spin?

1 A.Otero-de-la-Roza, M.A.Blanco, A. Martín Pendás, Víctor Luaña. Critic: a new program for the topological analysis of solid-state electron densities, Computer Physics Communications Volume 180, Issue 1, January 2009, Pages 157-166. DOI: 10.1016/j.cpc.2008.07.018

2 A.Otero-de-la-Roza, Erin R.Johnson, Víctor Luaña, Critic2: A program for real-space analysis of quantum chemical interactions in solids, Computer Physics Communications Volume 185, Issue 3, March 2014, Pages 1007-1018. DOI:10.1016/j.cpc.2013.10.026

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Yes, see e.g. the hydrogen molecule cation.

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    $\begingroup$ Can you share any reference about? Thanks. $\endgroup$
    – Camps ♦
    Apr 5, 2021 at 20:28
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    $\begingroup$ Reference for what? $\endgroup$
    – Susi Lehtola
    Apr 6, 2021 at 0:01
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    $\begingroup$ @Camps en.wikipedia.org/wiki/Dihydrogen_cation $\endgroup$
    – wzkchem5
    Jul 3, 2021 at 18:25
  • $\begingroup$ So yeah, this is a paper from 1927. link.springer.com/article/10.1007%2FBF01504875 $\endgroup$
    – Susi Lehtola
    Jul 5, 2021 at 6:05
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    $\begingroup$ @Camps Sorry I didn't read your question carefully. The absence of bond critical point does not mean a complete absence of bonding. Suppose you have a three-membered ring ABC, so that A-B, B-C and A-C are all bonded. Now you increase the ABC angle until it reaches 180 degrees. At ABC=180 degrees, there is certainly no bond critical point between A and C. So there must be an angle at which the bond critical point disappears abruptly. But you cannot say the A-C bonding suddenly disappears at that angle. There is no qualitative change of electronic structure at that point. $\endgroup$
    – wzkchem5
    Jul 5, 2021 at 17:05

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