Microscopic Equations of State, Thermodynamical Properties, and System Collapse: Application of the Dynamical Equation of Period Vectors in Crystal Structure Prediction under External Temperature and Stress/Pressure

In crystal structure prediction, Newton's second law can always be applied to particles (atoms or ions) in a cell to determine their positions. However the values of the period vectors (crystal cell edge vectors) should be determined as well. Here we applied the dynamical equation of period vectors derived recently based on Newton's laws (doi:/10.1139/cjp-2014-0518) for that purpose, where the period vectors are driven by the imbalance between the internal and external pressures/stresses. Since the internal stress has both a full kinetic energy term and a full interaction term, the in uences of both external temperature and stress/pressure on crystal structures can be calculated in the form of microscopic equations of state. Then thermodynamical properties and processes were presented, with thermal expansion shown even if only harmonic vibrations for phonons are considered. Finally, crystal system collapse due to temperature and/or stress/pressure change is discussed.