# Number of water molecules in a box to simulate liquid water [closed]

I have seen that depending on the simulation approach, different number of water molecules are employed for studying liquid water properties.

32 in early DFT calculations
The electronic structure of liquid water within density-functional theory The Journal of Chemical Physics 123, 014501 (2005); https://doi.org/10.1063/1.1940612

216 in early classical molecular dynamic simulations

Molecular Dynamics Study of Liquid Water The Journal of Chemical Physics 55, 3336 (1971); https://doi.org/10.1063/1.1676585

256 I'm not sure if this number of water molecules is just for computational reasons $2^8$= 256

So I'm wondering, What is the reasoning behind them?,

Edit: In other words, If I want to obtain the ambient density of the system and I don't know anything about it (number of molecules or the size of the simulation box, assuming cubic), what should I do?

As a starting point, the only thing I know is that the size of the simulation box should be chosen considering a number density that corresponds to the experimental density of the system.

• Different simulation methods require different amounts of computational horsepower. You simulate as many particles as you can afford to... – Jon Custer Apr 12 '18 at 19:58