Constrained magnetic calculations strategy
Posted: Thu Sep 28, 2023 10:00 am
Dear all,
A student of mine is using constrained magnetic calculations to try to stabilize magnetic configurations (which are collinear, and close to the ground state ironically) of an extremely pesky heterostructure with Cr2O3 and a strong spin-orbit-coupled heavy metal. I know that the common strategy for non collinear constrained calculations in VASP is to start with a small (lambda=1-10) lambda value to avoid instabilities, and then gradually increase lambda starting from a converged WAVECAR until the penalty energy is sufficiently small.
However, my understanding is that this is assuming that with the initial, small lambda value, the magnetic moments converged to the desired directions specified by M_CONSTR, just with a larger penalty energy than is desired. However, the student is getting the opposite issue, where when she sets lambda to 5 or 10, the final penalty energy is tiny (10^-9 eV), BUT the moments are nowhere near the desired magnetic configuration, and instead many are flipped with respect to M_CONSTR. It almost is as if the constrained formalism is just not kicking in.
In this situation, my intuition would be to actually attempt setting lambda to a much larger value; I know that people generally say that Ep is inversely proportional to lambda, but my understanding is that this is only in the limit where the moments are very close to their desired direction (small theta limit). Just looking at the functional from of the penalty energy, I would assume that when the moments are far away from their desired direction, a larger lambda will give a larger penalty energy and thus be more likely to force the moments to do what we want. Is this intuition correct/would you suggest this (I know it might fail due to instability issues or other problems)? Or are there any other strategies to use if the normal constrained magnetic procedure does not result in the moments pointing where you want them?
Thank you in advance!
A student of mine is using constrained magnetic calculations to try to stabilize magnetic configurations (which are collinear, and close to the ground state ironically) of an extremely pesky heterostructure with Cr2O3 and a strong spin-orbit-coupled heavy metal. I know that the common strategy for non collinear constrained calculations in VASP is to start with a small (lambda=1-10) lambda value to avoid instabilities, and then gradually increase lambda starting from a converged WAVECAR until the penalty energy is sufficiently small.
However, my understanding is that this is assuming that with the initial, small lambda value, the magnetic moments converged to the desired directions specified by M_CONSTR, just with a larger penalty energy than is desired. However, the student is getting the opposite issue, where when she sets lambda to 5 or 10, the final penalty energy is tiny (10^-9 eV), BUT the moments are nowhere near the desired magnetic configuration, and instead many are flipped with respect to M_CONSTR. It almost is as if the constrained formalism is just not kicking in.
In this situation, my intuition would be to actually attempt setting lambda to a much larger value; I know that people generally say that Ep is inversely proportional to lambda, but my understanding is that this is only in the limit where the moments are very close to their desired direction (small theta limit). Just looking at the functional from of the penalty energy, I would assume that when the moments are far away from their desired direction, a larger lambda will give a larger penalty energy and thus be more likely to force the moments to do what we want. Is this intuition correct/would you suggest this (I know it might fail due to instability issues or other problems)? Or are there any other strategies to use if the normal constrained magnetic procedure does not result in the moments pointing where you want them?
Thank you in advance!