However, the quantity of interest is the ratio of the partition function of the chain in
the presence of an external field to the one in absence of it,
ʃ Drexp (-ffi∕0[r] - ∕3f∕1[r, w])
ʃ Drexp (—χ3C70[r])
(8.18)
This can be further approximated as
Q[w] ≈ — / ⅛m-+1 [exp(-Δsw(rjvs))Φ(rjvs - rNs-i) exp(-∆sw(rjvs-ι))
Φ(r7vs-ι - r7vs-2)∙∙∙ exp(-Δsw(rι))Φ(rι - r0) exp(-∆sw(ro))]. (8.19)
Or,
Q[w] = — / dr g(r, N- [w]) = — / dr q(γ, N — s; [w])g(r, s; [w]), (8.20)
where
q(r, 0; [w]) = exp(-∆sw(r))
(8.21)
q(r, s + ∆s; [w]) = exp(-∆sw(r)) ʃ dr'Φ(r — rl)q(rl, s; [w])
Physically, g(r, s; [w]) represents the statistical weight for a chain of length ‘s’ with its
end at position r. This is similar to the probability density, pŋ(r, s) in the absence of
the external field. Using this analogy, eqn. 8.21 is a Chapman-Kolmogorov equation
in the presence of an external field, and the resulting Fokker-Planck equation is given
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