I’ve come across a handful of tutorials on free energy perturbation (FEP) that use a pme interpolation order of 6, like the one from mdtutorials. It sets the interpolation order value in the GROMACS .mdp file with the following:
pme-order = 6
I have found other, different types of FEP tutorials that use interpolation orders of 4, like the ones linked below:
http://pmx.mpibpc.mpg.de/tutorial.html
I have a general understanding of how PME works and some familiarity with FEP simulations and alchemical transformations, but have not run FEP simulations myself.
Could someone explain to me if and/or why higher order interpolation is needed for some or all types of FEP simulations?
Thank you!
PME interpolation order determines the number of grid points used to interpolate the long-range electrostatics in GROMACS simulations. The default value of 4 or 5 is sufficient for most systems. However, higher interpolation orders (up to 7) may be necessary for systems with larger unit cells or larger periodicity in the z-dimension, where the default values may not be accurate enough.
In terms of FEP simulations, higher interpolation orders may be useful to improve the accuracy of free energy calculations, especially for systems with a high degree of charge transfer or polarization, where the electrostatic energy plays a significant role. However, it should be noted that higher interpolation orders require more computational resources and longer simulation times. In general, it is recommended to use the default interpolation order unless it is necessary to increase it based on system-specific considerations.
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