  This article is a continuation of the past three papers, arXiv:1711.04976
(2017), arXiv:1809.04291 (2018), and arXiv:2006.09673v2 (2020), in which
configurations of the molecules around a vacancy in solid CO2 with the Pa3
structure (phase I) were calculated by the Monte Carlo (MC) simulation
technique at T < 200 K and at a nominal pressure of P = 1 atm using lattice
constants determined in reference to experimental data. For the intermolecular
potential, the Kihara model of a rod-shape core with zero diameter was used.
For theoretical consistency, however, the lattice constant should be determined
by a pressure-constant MC simulation, i.e. by the NPT simulation. It was
anticipated that the NPT simulation, successful for monoatomic molecular
fluids, might not work straightforwardly for solid CO2 because of the
non-spherical molecular shape and interactions. In fact, it was found in the
present work that the protocol of the standard NPT simulation had to be
modified for solid CO2 to achieve an equilibrium lattice constant, given an
external pressure, within practical computer time and capacity. The modified
protocol was used to calculate the P-V relation at 1 bar and from 2 to 10 GPa
at which a structural phase transition is known to take place. The temperature
was fixed at 200 K. As a result, the calculated P-V relation was similar to
those in previous theoretical works, the molar volume being roughly 0.5 %
smaller than experimental data.
