We extend the solid-state nudged elastic band method to handle a non-conserved order parameter – in particular , magnetization , that couples to volume and leads to many observed effects in magnetic systems .
We apply this formalism to the well-studied magneto-volume collapse during the pressure-induced transformation in iron – from ferromagnetic body-centered cubic ( bcc ) austenite to hexagonal close-packed ( hcp ) martensite .
We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa , with the transition-state enthalpy of 156 meV/Fe at this pressure .
A discontinuity in magnetization and coherent stress occurs at the transition state , which has a form of a cusp on the potential-energy surface ( yet all the atomic and cell degrees of freedom are continuous ) ; the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples .
Our results agree with experiments , but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom .