fix is used to update the atom positions and velocities by time integration. By assuming fix NPT you can control the pressure and temperature simultaneously. To obtain stress you have to compute stress. The Tdamp or Pdamp determines how rapidly the temperature or pressure is relaxed. For example
fix 1 all nvt 100 100 10
means that the temperature will be 100(temperature units) after 10 time units. You can not apply NPT with non periodic b.c.