question about fix npt command

Dear users,

For NPT, it means isobaric-isothermal case, but why can we write T start, T stop, P start, P stop in the command? Isn’t NPT supposed to be temperature and pressure are both constant? I search the internet but still cannot find why.
Thank you.

Miao-Chun Wang

Dear users,

For NPT, it means isobaric-isothermal case, but why can we write T start,
T stop, P start, P stop in the command?

​first of all. using fix npt doesn't *automatically* mean that you have an
NPT ensemble. this is a common mistake that people make. it has been
discussed many times on this mailing list.
​but if you use
​fix npt​
on a homogeneous, equilibrated, fully periodic system _and do no other​
manipulations of the system_ then you will sample the NPT ensemble.

however, the fix can also be used to establish certain non-equilibrium
​conditions, e.g. a temperature ramp or a pressure ramp.

Isn't NPT supposed to be temperature and pressure are both constant? I

search the internet but still cannot find why.

​it would be better to look into a suitably detailed text book on MD and
statistical mechanics.​ pressure and temperature in such systems are
constant only in the limit of an infinite number of particles. what you are
looking at in any statistical mechanical ensemble are the ensemble averages
(over particles and time) and fluctuations, hence the requirement for a
system that is and remains in equilibrium.

axel.