Subject: Why WHAM?

From: [email-masked]
Date: Wed, 26 Jan 2000 17:39:10 -0500 (EST)

I've read Kumar's paper [1] on the WHAM method for combining multiple
histograms, but it's still not clear to me what's exactly the
advantage of this method over the original umbrella sampling method
proposed by Torrie and Valleau?  In Chandler's textbook [2] (for
example) he describes a way of using umbrella potentials to construct
a curve of density of states as a function of a reaction coordinate (I
think this is basically Torrie and Valleau's method).  The approach is
very straightforward: the curves obtained with the different window
potentials are spliced by shifting them around vertically to produce a
smooth curve (p. 172).  This may be a bit crude compared to WHAM, but
at least it is completely transparent.  It is true that this "naive
splicing" procedure would be difficult to carry out if one wanted to
produce a density of states *surface* (in contrast to a curve) as a
function of two reaction coordinates, and altogether impossible when
the number of reaction coordinates is greater than 2.  But, at any
rate, naively it seems to me that if one has a single reaction
coordinate, then the simple procedure that Chandler describes should
suffice.  My concern is that, in such relatively simple cases, the
additional complexity of WHAM may not be justified by its advantages.
Am I missing something?  I would very much like to read your opinions.

Regards,

Kynn Jones

[1] Kumar, S. et al., J. Comp. Chem. 13(8):1011-1021, 1992
[2] Chandler, D. "Introduction to Modern Statistical Mechanics",
    (1987) pp. 169-175