3.1 Temperature and pressure

Several quantities in THERMO_PW can be calculated as a function of temperature at zero pressure or at selected pressures. Moreover the equilibrium geometry can be searched at fixed pressure minimizing the enthalpy instead of the energy. In some cases it is also possible to plot some quantities as a function of pressure at zero temperature or at selected temperatures. There is therefore the necessity to specify uniform meshes of temperatures (pressures) or to choose selected temperatures (pressures). The meshes are specified giving the minimum and maximum temperatures (pressures) and the interval between temperature (pressure) points. Selected temperatures (pressures) are instead defined giving their number and their values. The code will choose these temperatures (pressures) on the uniform mesh taking the point closest to the specified temperature (or pressure). For the options where these features are active, the values of temperature and pressure are controlled by the following variables:

tmin       : minimum temperature for the mesh of temperatures.
             Default: real 1 K
tmax       : maximum temperature for the mesh of temperatures.
             Default: real 800 K
deltat     : interval between two temperatures. Be careful 
             with this value because it is used also to 
             compute temperature derivatives numerically. 
             Too small or too large values could give 
             inaccurate anharmonic properties.
             Default: real 3 K
ntemp      : number of temperatures.
             Default: integer determined from previous data
pressure   : The external pressure. The crystal parameters are
             calculated minimizing the enthalpy at this 
             pressure. Given in kbar units. 
             Default: real 0.0 kbar
pmin       : minimum pressure for the mesh of pressures.
             Default: real -50.0 kbar
pmax       : maximum pressure for the mesh of pressures.
             Default: real 100.0 kbar
deltap     : Interval between two pressures in the mesh of pressures.
             Default: real 1.0 kbar
ntemp_plot : number of temperatures in the plots where the 
             temperature is a parameter. When 0 these plots are 
             not produced.
             Default: integer 0
temp_plot(ntemp_plot)  : A real array of dimension ntemp_plot with 
             the values of the temperature.
             No Default value, must be given (in K) when 
             ntemp_plot is not zero. 
npress_plot : number of pressures in the plots where the 
             pressure is a parameter. When 0 these plots are 
             not produced.
             Default: integer 0
press_plot(npress_plot) : A real array of dimension npress_plot 
             with the values of the pressure.
             No Default value, must be given (in kbar) when 
             npress_plot is not zero.
nvol_plot : number of volumes in the plots where the volume is a 
            parameter. When 0 these plots are not produced.
            Default: integer 0
ivol_plot(nvol_plot) : an integer array with the volumes to 
            consider in the plots where the volume is a parameter. 
            The chosen volumes can be only among one of the ngeo(1) 
            geometries.
            No Default value, must be given when nvol_plot 
            is not zero.

Note that when you fix the external pressure, the geometries chosen to fit the enthalpy must be about the minimum geometry at that pressure. Similarly, when using pressure ranges, the number of simulated geometries must be large enough to cover the selected range of pressures, otherwise the plotted quantities might be inaccurate.