How to control charge state and magnetism of NO ligand in spin-polarized ABACUS calculations (NO⁺ vs NO⁰/NO⁻ issue in relax, DFT+U unstable) #6824
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I am performing spin-polarized DFT calculations with ABACUS (LCAO, PBE) on a charged Cu–Histidine–Fe(CN)₅NO surface/cluster model and trying to stabilize the NO⁺ ligand state.
Chemically, NO is expected to be linear and non-magnetic (NO⁺). While such NO⁺-like solutions can be obtained in SCF calculations, during geometry optimization some NO ligands spontaneously bend and acquire finite spin density, indicating a transition toward NO⁰/NO⁻-like states. This occurs even when the total electron number and total spin are well defined and converged.
At present, I cannot independently control or constrain the charge and magnetic moment of individual NO ligands:
fixing total spin does not prevent ligand-centered spin localization,
relaxation can drive the system to a different self-consistent electronic branch,
and DFT+U cannot be used, as it consistently fails to converge in this system.
I would appreciate any guidance on whether ABACUS supports (or recommends) methods to constrain local magnetic moments or ligand charge/spin states, or best-practice workflows to avoid undesired electronic-state switching during spin-polarized geometry optimization.
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