Function Description
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	Name: Z_D_SIG 
	Function: Sigmoidal of dimer 
	Parameters: r s c d a b P T 

Notes
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	Gary Shaw's deltaG unfolding for dimers

C-code for the function
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	b = 8.314 * a[7];
	c = exp((0.0 - (a[4] + a[5] * x)) / b);
	d = a[2] + a[3] * x;
	e = (a[0] + a[1] * x) - d;
	f = sqrt(c * c + 8.0 * a[6] * c);
	answer = (e * (f - c)) / (4.0 * a[6]) + d;