Phonon calculations in CASTEP can be used to evaluate the temperature dependence of the energy, entropy, free energy and lattice heat capacity of a crystal in a quasiharmonic approximation. These results can be used to compare with experimental data (for example on heat capacity measurements), to predict phase stability of different structural modifications, etc. (see Baroni et al., 2001). Warren et al. demonstrated applicability of CASTEP to theoretical studies of phase transitions (Warren and Ackland, 1996 and Warren et al., 1998).
An example of the CASTEP output for thermodynamic properties is shown in Figure 1. All energy-related properties are plotted on one graph, and the calculated value of the zero-point energy is included. The heat capacity is plotted separately on the right.
The entropy is presented as an S*T product to allow comparison with the energy.
Thermodynamic properties calculated using the quasiharmonic approximation can be inaccurate for temperatures comparable to the Debye temperature.
To create a thermodynamic properties chart
Analyzing CASTEP results
Thermodynamic properties selection - CASTEP Analysis dialog
Results file selector