Gamma p

gamma_p sets a "bulk modulus"-like quantity that determines, together with tau_p, the strength of the coupling to the barostat. In TurboGAP, gamma_p is given in units of the inverse compressibility of water, [math]\displaystyle{ 1 / (4.5 \times 10^5 \text{ bar}^{-1}) }[/math]. While this may seem complicated, it provides a convenient and intuitive way to select gamma_p in terms of how incompressible (or "rigid") one expects the system under study to be, compared to water. For instance, for typical solids [math]\displaystyle{ \gamma_P \gg 1 }[/math] whereas for gasses [math]\displaystyle{ \gamma_P \ll 1 }[/math]. For most organic and molecular liquids [math]\displaystyle{ \gamma_P \approx 1 }[/math] should be a suitable choice. A simple way to select the optimal gamma_p is to divide the bulk modulus of the system under study by the bulk modulus of water.

Summary

Summary for gamma_p keyword

Required/optional	Type	Accepted values	Default	See also
Optional	Real	Any positive real	1.	barostat, p_end, tau_p, gamma_p

Example

barostat = "berendsen"
barostat_sym = "iso"
p_beg = 10. # in bar
p_end = 10. # in bar
tau_p = 100. # in fs
gamma_p = 10. # in units of the inverse compressibility of water