I'm trying to calculate film thickness of adsorbed molecules (in nm scale) and this variable came up.
It's my first time trying this kind of calculation, so I have no clue on the variable.
What does correlation length mean and how can I calculate it for a fluid?
All I could find out about correlation length was that it is a variable used to describe a surface roughness and I don't think this was the one I was looking for.
The following explanation might be partially redundant, since I don't know your background:
You might have seen pair distribution functions, also known as pair correlation functions, which are a function of the separation distance between atoms or molecules. You can calculate them by writing a script or using built-in implementations in LAMMPS or Gromacs, or whatever molecular simulation software you use. These functions have sharp peaks (i.e. highly structured) for solids, while for gasses they tend to have one small peak and then decay to unity at longer distances. Fluids show an intermediate behavior with multiple peaks, that are more smeared out as the pair-distance increases. These peaks indicate that the probability of finding an atom or molecule at a certain distance is more probably than you would expect based on the average density of the material. This increased (or decreased in case of a trough) probability is caused by the correlation between neighboring atoms or molecules. The correlation length would be the length at which the structure in the pair distribution function has disappeared. However, the decay might be exponential and for practical purposes one has to decide how much the structure in the function need to be decayed to call it uncorrelated. For example, the correlation length can be defined as the length which the magnitude of the peaks and troughs is less than 10% of the maximum peak height. A good way to estimate this is by fitting the magnitudes of the peaks in the correlation function.
Thank you for answering my question.
It helps a lot to understand the concept.
My major is chemical engineering and I'm currently studying porous materials.
However, I am not using any molecular simulation software.
Is there any way to calculate the value using other reference values with hand calculation?
After computing the correlation function, there is some freedom in how you define and estimate the correlation length from the function. So at that point you can do without complicated calculations. However, without a correlation function you have very little to go by to estimate the correlation length by hand, if that is what you meant.
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