[CP2K-user] [CP2K:13141] spin of unpaired electrons

Lucas Lodeiro eluni... at gmail.com
Sat Apr 18 01:24:51 UTC 2020

```Hi Pavan,

I am not an expert on CP2K code, but in a theoretical point of view what
you want to calculate is the same state, just in a different micro-state.
The case you show, or what ever case of duplet spin multiplicity has two
micro-states (in the spin freedom degree), and both have the same energy
(degenerated) and are described by the same spectroscopic term, but with
different z spin projection. In both cases the quantum state for all
properties is the same (except for Ŝ_z), then most of program, for
simplicity, uses the alpha channel to project the spin.
For higher spin multiplicities, you have more micro-states for the same
spectroscopic term, but is complex in some cases. The triplet example, you
have 3 micro-states, S_z=+1,0,-1. The former and the latter are a single
determinant and the only difference is the channels of the unpaired
electrons (two), is a simple case as duplet multiplicity, but, the S_z=0
case is complex, due to this state is a linear combination of two
determinants, and needs multi reference description. Then the full spin
projection is a simpler case.

Only if you use a magnetic field, the degeneration is broken, and both
cases you mention are differents.

Regards - Lucas Lodeiro

El jue., 16 abr. 2020 a las 17:38, pavan kumar behara (<
pavan... at gmail.com>) escribió:

> Hello CP2K developers,
>
> I notice that excess electrons are always assigned to alpha channel, is it
> possible to generate a configuration where I can have more beta electrons?
> Let's consider a minimal example of hydrogen molecule with excess charge,
> H-H(-), making it a 3-electron system. I can always calculate [2 spin-up, 1
> spin-down] configuration
>
> ↑
>
> ↑*↓*
>
>
> but never [1 spin-up, 2 spin-down]
>
> ↓
>
> ↓*↑*
>
>
> I started with a spin-down hydrogen as initial atomic guess but the
> density matrix is re-scaled before starting the SCF iterations making it 2
> alpha and 1 beta.
>
> Is there a way to get the second configuration or there is a reason not to
> do so? I attached the inputs and output for your reference.
>
> Any help is appreciated, thank you very much for your time.
>
>
> Best regards,
> Pavan.
>
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