Local volume as a robust structural measure and its connection to icosahedral content in a model binary amorphous system

Molecular dynamics simulations are performed for a model binary glass to investigate the temperature dependence of the thermally driven localized structural excitations mediating glass relaxation. This is done under fixed zero pressure by increasing the temperature from zero to values close to and beyond the glass transition regime. It is found that for a given ramp rate, fixed pressure simulations admit a similar amount of structural relaxation as fixed volume simulations (Acta Materialia 143 (2018) 205–213), but at a reduced temperature scale corresponding to a reduced glass transition temperature. Furthermore, analysis of the volume evolution under fixed pressure, and stress evolution under fixed volume, demonstrates an equivalence between NPT and NVT ensemble molecular dynamics simulations in achieving a particular structural state. The observed relaxation induces a local densification which may be quantitatively connected to the creation of icosahedral structure that minimizes both bond energy frustration and maximizes local atomic packing. Together these results establish a robust link between the structural state of an amorphous solid and its free volume content, both of which now directly relate to the degree of relaxation.