How to Simulate a Mixed-Spin State of HS and IS with Quantum Admixture?

[bracci47]

Hello EasySpin Experts,

I have a somewhat advanced question about simulating mixed-spin states, specifically dealing with the quantum admixture of high-spin (HS, S=5/2) and intermediate-spin (IS, S=3/2) states.

Background:
I came across a paper by M. M. Maltempo and T. H. Moss (DOI 10.1017/S0033583500002407), which suggests that the quantum admixture of HS and IS states could have important implications in heme proteins. My aim is to simulate this phenomenon using EasySpin to understand its characteristics better and possibly validate experimental data.

Question:
Could anyone guide me through the Hamiltonian structure and EasySpin function calls that would be needed to simulate a CW-EPR of a mixed HS-IS state?

Are there specific Hamiltonian terms I should include to describe the HS and IS states adequately?
How do I integrate these terms to reflect the quantum admixture?
Any additional parameters or function calls that I need to be aware of?

Best regards,
Maruan

[DanielKlose]

Hi Maruan,

as I understand your question, the first step would be to simulate solid state cw EPR spectra with the two spin state of Fe, so S=3/2 and S=5/2.
The EasySpin example with high spin Cr(III) should be a good starting point for you, you can find it here:
https://easyspin.org/easyspin/examples/ ... te/criii.m

Then when you're fine with the two individual models, you can combine the two spin systems, Sys1 and Sys2, by simulating with the solid state function pepper using pepper({Sys1,Sys2},Exp).
If the two electron spins are coupled in your case, you can see how to describe this in the spin systems here:
https://easyspin.org/easyspin/documenta ... n.html#eei

Best wishes!

[bracci47]

Hi, and thank you for your response.

I appreciate the guidance on simulating individual high-spin and intermediate-spin states. However, I believe there may be a misunderstanding about the core of my question.

My interest is not in simulating two separate electron spin systems for Fe with S=3/2 and S=5/2. Rather, I'm focused on understanding a single electron spin system that exists in a quantum mechanical mixed-spin state, essentially a (variable) superposition of the high-spin (HS, S=5/2) and intermediate-spin (IS, S=3/2) states.

I am wondering how to construct the Hamiltonian (I am not sure whether it's even possible in EasySpin, but it would be of great help to undertand the problem) to reflect this quantum admixture of HS and IS states. If EasySpin cannot achieve this, are there alternative computational approaches or software that you'd recommend?

Thank you for your time, and I look forward to any further insights.

Best regards,
Maruan

[DanielKlose]

Hi Maruan,

the third point mentioned should take you there:
You have two options:
1) you can either define a Spin System with two electron spins coupled by exchange and dipolar electron-electron interactions
2) or you can specify the larger Group Spin, S = S1 + S2, and a zero-field splitting,
both descriptions are in principle equivalent and describe the state mixing (i.e. interaction).
These options to define a Spin System are described in the documentation under https://easyspin.org/easyspin/documenta ... ystem.html

Best wishes!

[christiansen]

Quick input from my size: ES relies on having a quantum state represented in an uncoupled m_J basis, i.e. each site should have a well-defined spin quantum number. A state with quantum mixing of HS and IS states does not have a well-defined value of S as far as I am aware, thus I imagine ES is not well suite for describing such a system. However, I am not expert, so please correct me if I am wrong.

I bet CASSCF would do an okay job describing your system, but I am not a computational specialist by any means.