Summary
The NPT ensemble (isothermal-isobaric ensemble) maintains number of particles, pressure, and temperature in a molecular dynamics simulation. Spontaneous changes in an NPT ensemble cannot lead to increases in Gibbs free energy (1). The reservoir acts as both a thermostat and a barostat.
1.
Understanding Molecular Simulation. 2002; Available from: https://doi.org/10.1016/b978-0-12-267351-1.x5000-7