This variable selects how the fictitious mass W of the cell (barostat) is determined
in the Parrinello-Rahman equation of motion of the cell (see CellDynamics).
The value of W only affects the dynamical trajectory of the cell, not the
time-averaged or equilibrium properties (Parrinello and Rahman, J. Appl. Phys. 52, 7182 (1981)).
Options:
parrinello_rahman:
The fictitious cell mass is set to the total ionic mass, W = sum_i m_i. This is a
simple, robust choice used in variable-cell ab initio molecular dynamics
(Parrinello and Rahman, J. Appl. Phys. 52, 7182 (1981); Wentzcovitch, PRB 44, 2358 (1991)).
martyna_tobias_klein:
The fictitious cell mass follows Martyna, Tobias and Klein, J. Chem. Phys. 101, 4177 (1994):
W = (N_f + d) k_B T tau_b^2, where N_f is the number of ionic degrees of freedom, d the
periodic dimension, T the target temperature, and tau_b the barostat time constant
(BarostatTimeConstant). This requires a finite temperature, i.e. a Thermostat.
Since the cell is here propagated through the lattice matrix h (length units), the mass is
mapped to W = (N_f + d) k_B T tau_b^2 / V^(2/3), so that W has units of mass (V is the
cell volume, providing the natural area scale of the cell).