The Nature of Fluid Flow - Fluid Properties at Nano/Meso Scale: A Numerical Treatment - page 21

suitable number of time steps. The simplest form of temperature control is to
scale the velocities periodically, but this is not a truly isothermal method and
must be removed before the properties are collected.
Although energy is considered to be conserved, there will be slight fluctua-
tions and the possibility of a small drift due to truncation and rounding errors
from the calculations.
This type of ensemble is useful for predicting thermodynamic response
functions.
Canonical Ensemble (NVT) . a constant number of particles, volume and tem-
perature. As in the microcanonical ensemble, during the initialization stage the
velocities are scaled to the desired value for the set temperature. Although use-
ful for initialization, velocity scaling is not suitable to use as a control for a
simulation as it is crude and not a truly isothermal method. Therefore, other
thermostatic methods must be used to apply the temperature control, which will
be explained in detail in Chapter 3. This ensemble is used to perform confor-
mational (spatial arrangements of a molecule) searches of models evaluated in
a vacuum without periodic boundary conditions. Even when periodic boundary
conditions are used, this ensemble can be useful if pressure is not a significant
factor, as the constant temperature and volume provides less perturbation due
to the absence of pressure coupling.
Isobaric-Isothermal Ensemble (NPT and NST).
- NPT: a constant number of particles, pressure and temperature. Tempera-
ture is controlled using one of the thermostatic schemes detailed in Chap-
ter 3 and the pressure is controlled by varying the volume of the system us-
ing the Berendsen, Anderson or the Parrinello-Rahman schemes [13]. The
Berendsen and Anderson schemes work by varying the size of the system,
while the Parrinello-Rahman scheme is also capable of varying the shape of
the system.
- NST: a constant number of particles, stress and temperature. This is an ex-
tension to the constant pressure ensemble, which adds extra control on the
stress components xx , yy , zz , xy , yz and zx .
Both methods are mainly used in structural applications. NST can be used to
evaluate stress/strain relationships and NPT is generally used when the correct
pressure, volume and temperature are important.
NPH and NSH.
- NPH: a constant number of particles, pressure and enthalpy. This ensem-
ble is similar to the NVT ensemble, only the size on the cell is allowed
to vary.