Re: Re: Parameters for NH3, NH4+, and CO2

From: Jeffrey J. Potoff (jpotoff_at_chem1.eng.wayne.edu)
Date: Wed Nov 28 2007 - 21:34:29 CST

On Wed, 28 Nov 2007, Ilya Chorny wrote:

> Anyone know where I can find forcefield parameters for the above molecules.
> i.e. charges and atom types. I would steel them from the protein i..e LYS
> but I do not know the charges. Any ideas?
>
> Thanks,

I can't help you with NH3 or NH4+, but for CO2, we developed a very
accurate force field a few years ago [AICHE Journal (2001), 47,
1676-1682]. For NH3, I would search J. Chem. Phys., J. Phys. Chem. B and
Fluid Phase Equilibria. I'm sure someone has already done it.

If you intend to generate your own parameters, charges are relatively easy
if you have access to Gaussian. Optimize at the HF/6-31g+(d,p) level and
use the "pop=CHELPG" keyword. The charges that result will be about the
best you can get without tuning them empirically. For small molecules, we
generally increase the magnitude of the charges that come out of this
methodology by 10-20% in our empirical fitting process. As for the LJ
parameters, "borrowing" them from larger molecules is likely not going to
work, since you are talking about small polar molecules where the N atom
is in a very different bonding environment than wherever you took the
parameters from. If you don't find something satifactory in the
literature, you will have to fit the epsilons and sigmas to reproduce the
liquid density and heat of vaporization as a minimum. Again, this will
get you in the ballpark, but you can do even better with small molecules
if you fit your parameters to reproduce phase behavior and vapor pressure.

Regards,
         Jeff

======================================================================
Jeffrey J. Potoff jpotoff_at_chem1.eng.wayne.edu
Associate Professor Wayne State University
Department of Chemical Engineering and Materials Science
5050 Anthony Wayne Dr Phone:(313)577-9357
Detroit, MI 48202 Fax: (313)578-5815
http://potoff1.eng.wayne.edu
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