老师好，我前段时间学习ASE中的NPT模块（NH热浴+PR压浴）时遇到个参数问题：
pfactor:
A constant in the barostat differential equation. If a characteristic barostat timescale of ptime is desired, set pfactor to ptime^2 * B (where B is the Bulk Modulus). Set to None to disable the barostat. Typical metallic bulk moduli are of the order of 100 GPa or 0.6 eV/?^3. Useful parameter values:
The ttime and pfactor are quite critical[4], too small values may cause instabilites and/or wrong fluctuations in T / p. Too large values cause an oscillation which is slow to die. Good values for the characteristic times seem to be 25 fs for ttime, and 75 fs for ptime (used to calculate pfactor), at least for bulk copper with 15000-200000 atoms. But this is not well tested, it is IMPORTANT to monitor the temperature and stress/pressure fluctuations.
我查了下体香bulk modulus和compressibility是倒数关系，比如水的压缩系数是4.5*10^-5 bar^-1，这也是GROMACS里压浴的常用参数。
我又看了其源代码中pfactor和ptime的转换关系如下，有点迷惑，到底该怎么设置呢？
def _calculateconstants(self):
"(Re)calculate some constants when pfactor, ttime or temperature have been changed."
n = self._getnatoms()
if self.ttime is None:
self.tfact = 0.0
else:
self.tfact = 2.0 / (3 * n * self.temperature *self.ttime * self.ttime)
if self.pfactor_given is None:
self.pfact = 0.0
else:
self.pfact = 1.0 / (self.pfactor_given * linalg.det(self._getbox()))
# self.pfact = 1.0/(n * self.temperature * self.ptime * self.ptime)
self.desiredEkin = 1.5 * (n - 1) * self.temperature