Pressure and Temperature
Why does the pressure go
negative? In a molecular dynamics
simulation the pressure is calculated using an equation due to Rudolph
Clausius (1822-1888) which has the form
PV = nRT -
W/3
where W is known as the Virial of Clausius, which
takes into account the influence of intermolecular forces. For a
dilute gas, W is small and we recover the normal Universal Gas Law.
However, in the Democritus simulations W is both significant and
positive - indicating that attractive forces are present. This means
that for a fixed volume and low temperature this term becomes dominant
and the pressure becomes negative. In other words, the attractive
intermolecular forces overcome the kinetic energy of the system, which
implies that the system cannot remain a gas at this volume. Thus you
should see that when the pressure becomes negative, colliding atoms
tend to stick together. In Nature negative pressure does not
arise, because the system contracts in volume and the virial reduces
in magnitude. This cannot happen in the simulation where the
simulation `box' is fixed in size, so the system must continue in a
state of `negative pressure'.
