TITLE

On the use of acceptance ratio methods in free energy calculations

AUTHOR(S)
Ferguson, David M.
PUB. DATE
December 1993
SOURCE
Journal of Chemical Physics;12/15/1993, Vol. 99 Issue 12, p10086
SOURCE TYPE
Academic Journal
DOC. TYPE
Article
ABSTRACT
Molecular dynamics free energy simulations have been performed on three solvated systems to examine the potential benefits of Bennett’s acceptance ratio (AR) method [Bennett, J. Comput. Phys. 22, 245 (1976)], as compared to standard statistical perturbation techniques (FEP). The AR sampling equation has been implemented along with double-ended FEP sampling to calculate the relative solvation free energy of neon/nothing, neon/TIP3P-water and phenol/benzene in periodic TIP3P water. A histogram method is also introduced to calculate the overlap in the double-ended samples 0 and 1 from the density functions, p0(ΔE) and p1(ΔE), allowing window increments (δλ) to be correlated with % overlap and accuracy in the calculations. This relatively new concept is based on the fact that accuracy in both AR and FEP methods can be shown to depend on the overlap. The results indicate that the AR methodology has some advantages for calculating free energies when sample sizes are limited and minimal overlap exists in the ensemble samples at each window, mainly in reductions to the statistical errors. However, as more extensive simulations revealed, if the overlap in the density functions is increased (through reductions in window sizes), the simple FEP averages and variances approach near parity with the AR ‘‘best estimate’’ results, indicating that the methods are effectively the same; with the caveat of adequate sampling.
ACCESSION #
7629368

 

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