ABF Free Energy Calculation

From: Joey Farrell (jpfarrell_at_uchicago.edu)
Date: Tue Oct 04 2022 - 01:12:52 CDT

Hello all,

I have three questions regarding how to properly use ABF to calculate Keq after reading the tutorial and searching the mailing list.

  1. When calculating ∆G via discrete integral, should one only consider bins with a sufficient zcount, or ought all bins on the reaction coordinate contribute? If there is a minimum/threshold zcount, is it a function of the fullSamples parameter, or is it a general, approximate threshold?
  2. In the ABF tutorial, the "unbound" state simulations have colvars which refer to the "bound" state structure as the reference structure. Should the centers of the harmonic restraints be updated such that there is no harmonic potential in frame 0 (i.e., an RMSD restraint originally with center 0 now has non-zero center since the reference structure is not itself)?
  3. Each colvars forceConstant is only realized by a proceeding simulation (i.e., there is no value for "k" until the next colvar's simulation, which then uses the previous as a harmonic restraint). What, then, is the "k" value for the last colvar that is run, since it is by definition never used as a harmonic restraint in another colvar's simulation? It seems it cannot be the case that k=0, as inductively you would never need the last simulation (since ∆G contribution is 0 for k=0 in my understanding).

Thank you very much; I greatly appreciate any help with these questions.


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