Re: pressure vs density tip3p

From: Roy Kimura (roy.kimura_at_bms.com)
Date: Mon Aug 27 2007 - 16:04:24 CDT

Actually, after typing this I realize the error bars bring my observed
value very close to the experimental value 0.997, but it still does not
explain the reported value for TIP3P (1.002) and the systematic trend I
observed with increasing pressure. Perhaps all I need to do is run longer?
Roy

Roy Kimura wrote:

> Dear NAMD group,
> I have been running boxes of pure water (216 TIP3P, starting box size
> ~ 18.83 angstrom/side periodic box) in the NPT ensemble using the
> Nose-Hoover Langevin piston method and temperature coupling (after 100
> ps equilibration with velocity rescaling up to 298K). I am running at
> 1.0 atm pressure and 298 K but when I compute the water density during
> a 500 ps run by dividing the mass of the simulated water (216 * 18.016
> / 6.022e23 grams) with the reported volume, I am getting an average
> value of about 0.978 +/- 0.018 which is much less than the values
> reported in the literature for TIP3P water (about 1.002, although
> experimentally it should be about 0.997). I checked the time plots of
> all relevant values (temperature, pressure, volume, density) and I am
> seeing no observable drift, i.e., the system looks to be settled down
> correctly). The average pressure value (-4.8 atm) was also reasonably
> close to the target pressure (1.0) given the fluctuations of about +/-
> 840 atm. After simulating at various pressures (1.0 atm, 50, 100 ,
> 500, 1000, 5000), I found the pressure vs density relation to be
> linear, and a regression line indicated that in order to obtain 0.997
> density, one needs to run at approximately 356 atm. Has anyone else
> seen this type of behavior? I have been checking the literature values
> and all papers I have found report the opposite trend (i.e., TIP3P
> appears to be slightly too dense at 1 atm 298K). I have thought that
> maybe the box size is too small -- however, a few of the oft-cited
> original TIP3P papers (1983, 1985) by the Jorgensen group has used the
> same system size. I would appreciate it if anyone has any insights
> into this. Thank you very much.
> Roy
>


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