[gmx-users] pressure coupling questions

[Q.Z.]

Dear All,
 I am new to md and gromacs. According to the manual and some the 
tutorial, one can set either temp coupling or pressure coupling or both 
at the same time. Does the temp coupling alone mean to use NVT ensemble 
and pressure alone NPT ensemble? What is it when use both coupling?
 If I turned off pressure coupling in my md but temp coupling on, I got 
about average pressure of -1000 bar. What does that mean? Is the density 
of my system too high? If it is, how should I change it?

Thanks for any input.

Q. Zou Ph D.
Indiana Univ.