  The interrelationship between a material's structure and its properties lies
at the heart of materials-related research. Finding how the changes of one
affect the other is of primary importance in theoretical and computational
materials studies. In this work, based on Hershfield nonequilibrium quantum
statistics and the mean-field approach with steady-state density functional
theory, we derive a first-principles method to calculate nonequilibrium effects
induced forces acting on atoms, enabling structure optimizations and molecular
dynamics simulations for molecular junctions under external biases. By applying
the method to a few molecular devices, we found that in general, the external
bias can induce profound nonequilibrium effects on both electronic/transport
properties and the geometric structure of these devices, and consequent changes
in electronic properties and geometric structure are closely interrelated.
Particularly, when the bias voltage is above 1.0 V, significant structure phase
changes could occur, causing dramatic changes in I-V characteristics and
vibrational spectra. These findings greatly broaden our understanding of
quantum electronic devices and provide a new avenue for discovering novel
transport phenomena at molecular scale.
