Subject: Re: van der Waals parameters

From: [email-masked] (Bill Ross )
Date: Thu, 9 Dec 1993 11:53:29 -0800

Date: Thu, 09 Dec 1993 08:12:32 -0500
	From: [email-masked] (Jeffrey L. Nauss)
	Subject: Re:  van der Waals parameters
	To: [email-masked] (Bill Ross )
	Message-id: <[email-masked]>
	Content-transfer-encoding: 7BIT

	Bill - 

	I disagree with your explanation of R and sigma.  Acoording to Moore
	in his book "Physical Chemistry" (4th edition) page 128, sigma for the
	Lennard- Jones potential is the value of r (distance) where the
	potential energy equals zero, i.e. where the curve crosses the x-axis
	other than where r approaches infinity.

	On the other hand, according to Hill in "Introduction to Statistical
	Thermodynamics", Appendix IV, pg 484, r* is the distance at the
	potential minimum.  Thus r* = sigma * 2^(1/6).

							Jeff Nauss

	************************************************************************
	*  Dr. Jeffrey L. Nauss             * Telephone: 513-556-0148          *
	*  Department of Chemistry          * Fax: 513-556-9239                *
	*  University of Cincinnati         * e-mail: [email-masked]  *
	*  Cincinnati, OH 45221-0172        *                                  *
	************************************************************************

Since the combining rule for sigma is the same as it would be
for 2 * R*, and the sigmas in Allen & Tildesly p 21 look like
2 * R*'s, I apparently made a wrong assumption. Thanks to Jeff
Nauss for the correction.

Bill Ross