Self-consistent versus non-self-consistent spin-spiral calculations of magnetic exchange interactions in VASP

We present an investigation of magnetic exchange interactions using the spin-spiral method as implemented in the code, comparing fully self-consistent (SC) total-energy calculations with non-self-consistent (NSC) band-energy evaluations within the same computational framework. Using representative 3 d ferromagnets (Fe, Co, Ni) and Mn-based full Heusler compounds, we compute magnon dispersion relations and extract real-space Heisenberg exchange parameters from the Fourier transformation of spin-spiral energies. Curie temperatures are subsequently estimated within the mean-field and random-phase approximations. The SC approach yields exchange parameters and magnon spectra in excellent agreement with experimental and previous theoretical data, confirming its quantitative reliability across different classes of magnetic systems. In contrast, the NSC approach based on a frozen-potential approximation exhibits systematic quantitative deviations: it overestimates spin-spiral energies and exchange couplings in high-moment systems (bcc Fe and Mn-based Heuslers) while underestimating them in low-moment fcc Ni. The magnitude of these discrepancies increases with the magnetic moment size and can exceed several hundred percent in high-moment compounds. Our results demonstrate that while NSC evaluations are computationally efficient, fully SC spin-spiral calculations are essential for obtaining quantitatively reliable exchange interactions within the projector augmented-wave framework.