Structural Prediction and Phase Transformation Mechanisms in Calcium at High Pressure

High-pressure phase transformations of Ca are studied using the metadynamics method to explore the anharmonic free-energy surface, together with a genetic algorithm structural search method to identify lowest enthalpy structures. Disagreement between theory and experiment regarding the structure of Ca in the pressure range 32--119 GPa is partially resolved by the demonstration of different phase transition behavior at 300 K from that at low temperatures. A new lowest enthalpy $I{4}_{1}/amd$ structure is obtained with both methods with an estimated superconducting critical temperature in agreement with experiment.