Free-energy calculations - Theoretical Biophysics Group

Substitute
where
Free-energy perturbation calculations Interpretation of the free energy perturbation equation
to the expression of ∆A.
P0(∆U) =
1
2 ∆U − 〈∆U〉0
√ exp −
2πσ 2σ2
σ 2
= ∆U 2
0 − 〈∆U〉 2
0
A Gaussian P0(∆U) obviates the need to carry out a numerical integration:
∆A = 〈∆U〉 0 − 1
2 βσ2
This analytical expression of ∆A has a broader significance:
The first term, which is simply equal to the average energy difference measured
in the reference state, can be either positive or negative.
The second term, which depends on fluctuations of ∆U, is always negative.
Chris Chipot Free-energy calculations