Lets' say I have a solid nanomaterial made with N atoms in a simulation box with a volume of V as shown below. In this case, the number density of my material is simply $\frac{N}{Volume\; of \; occupied\;3D\; space}$.
but what if I have a nanomaterial with $N$ atoms in the same simulation box, but $N_{1}$ atoms are in the solid phase and $N_{2}$ atoms are in the gas phase $\left( N_{1} + N_{2} = N \right)$.
How should I calculate the number density of my material in this case? Do I consider the whole box volume $V$? do I have to find an average over the whole material?
