  Molecular dynamics simulations are used to test when the particle-in-cell
(PIC) method applies to atmospheric pressure plasmas. It is found that PIC
applies only when the plasma density and macroparticle weight are sufficiently
small because of two effects associated with correlation heating. The first is
the physical effect of disorder-induced heating (DIH). This occurs if the
plasma density is large enough that a species (typically ions) is strongly
correlated in the sense that the Coulomb coupling parameter exceeds one. In
this situation, DIH causes ions to rapidly heat following ionization. PIC is
not well suited to capture DIH because doing so requires using a macroparticle
weight of one and a grid that well resolves the physical interparticle spacing.
These criteria render PIC intractable for macroscale domains. The second effect
is a numerical error due to Artificial Correlation Heating (ACH). ACH is like
DIH in that it is caused by the Coulomb repulsion between particles, but
differs in that it is a numerical effect caused by a macroparticle weight
larger than one. Like DIH, it is associated with strong correlations. However,
here the macroparticle coupling strength is found to scale as $\Gamma w^{2/3}$,
where $\Gamma$ is the physical coupling strength and $w$ is the macroparticle
weight. So even if the physical coupling strength of a species is small, as is
expected for electrons in atmospheric pressure plasmas, a sufficiently large
macroparticle weight can cause the macroparticles to be strongly coupled and
therefore heat due to ACH. Furthermore, it is shown that simulations in reduced
dimensions exacerbate these issues.
