R help - Jun 2006 - Function with upper/lower bound, shift,slope,curvature

Dear R Users,

I am looking for a function to use within optim s.t.
- f(x,a,b,c,d,e) where I am searching over parameter values a,b,c,d,e
- a is the lower bound (typically 1)
- b is the upper bound (typically 0)
- c,d,e give the function a shift, slope and curvature between a and b

Any good candidates for a non-linear optimisation problem known, either 
already built into R, or otherwise.

A sigmoid sort of gets half there, but I need some more control over the 
curvature and upper/lower bounds of the function.

sigm<-function(x,a,t) 0.5*(1+(1-exp((x-a)/t))/(1+exp((x-a)/t)))

Thanks in advance,
Tolga