# Pressure profile with long-range interaction into the system

I know that LAMMPS can calculate the pressure profile using Irving-Kirkwood method.
Is it applied for long-range forces too or only short-ranged ones?

What LAMMPS is capable of is documented in the manual. If something is not mentioned in the manual, chances are high that that is not a supported capability.
Please also know that it is difficult to discuss features based on “names” and “methods”, but rather in what commands you want to use.

Please also note that while there may be issues with using long-range coulomb during simulations for some kinds of on-the-fly analysis, it is often possible to substitute this with a cutoff coulomb for post-processing with good accuracy using the “rerun” command.

As someone who regularly works on PPPM, my colleague and I actually tried to tackle this for a while some time back.

Fundamentally, IK pressure works by integrating pairwise forces along the lines joining particles – long range forces work by having every charged particle exert a force on every other charged particle (via FFT calculation in PPPM). I hope you can see that making these two techniques work together would be a Very Difficult Achievement.

Furthermore, the physical quantities of interest are path integrals of the force flux field – which means that the force flux itself is not uniquely defined, as adding any divergence-less field gives the same result. (Or something like that; the Irving-Kirkwood paper and subsequent early papers make this quite clear.)

Long story short, this is a gap in theory, not just code. I would hesitate to quote a pressure profile on its own unless I had supporting evidence to tie a story together. For example, if I were examining two different compositions of a lipid bilayer in water, I would see how far each chemical species penetrates into the hydrophobic core.

Also, if you rerun a calculation of a static property with increasing electrostatic cutoff radii, you will find that the results either converge before the cutoff is a very large fraction of the box size, or they don’t.

If they do converge, then you can argue that you don’t need the long range electrostatic forces in that calculation any more because the system is homogeneous enough within the box for calculating that quantity.

If they don’t converge, then a referee would argue that your system simply isn’t big enough to calculate that quantity reliably anyway, since maybe a 2x box size would give you different results.