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Re: How to avoid overflow or underflow in mathematica?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg88674] Re: [mg88648] How to avoid overflow or underflow in mathematica?
*From*: "W_Craig Carter" <ccarter at mit.edu>
*Date*: Mon, 12 May 2008 04:46:47 -0400 (EDT)
*References*: <200805111914.PAA08842@smc.vnet.net>
Hello Quantum Fang,
You could use arbitrary precision and do something like this (although
I don't advise this approach):
fermiseries =
Normal[Series[1/(1 + E^((muShift) beta)), {beta, 0, 14}]];
colist = Simplify[CoefficientList[fermiseries, beta]]
and then write a function that calls N[beta, bigNumber] and N[mushift,
bigNumber] on the product of the two list until you have the accuracy
that you want.... This is a bad idea.
However your function, except for ((omega - mu - Ef)/KBT)
approximately 1, is the unit step function (0 if x<0, 1/2 x=0, 1 if
x>0). If you are interested in the physical aspects of this problem
then expand (omega - mu - ef) near 1/(kb T). In other words, scale
your energies with the thermal energy.
WCC
On Sun, May 11, 2008 at 3:14 PM, quantumfang <membrane at 163.com> wrote:
> It's well known that Mathematica can do evaluation to arbitrary precision. But I don't know how to avoid the following trouble in computing:
>
> Ef = 0.;
> mu=0.;
> KBT = 1.*10^-10;
> FL[omega_Real] := 1./(1. + E^((omega - mu - Ef)/KBT));
>
> the error message shows
>
> General::ovfl: Overflow occurred in computation. >>
> General::unfl: Underflow occurred in computation. >>
>
>
--
W. Craig Carter
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