[CP2K:1414] Re: Force field for fully flexible water

Laino Teodoro teodor... at gmail.com
Wed Oct 8 09:11:51 UTC 2008

Do you really believe that the fact that you have to use 0.05 fs  
while they claim
to use 0.75 fs is strange? if you use proper units you will see that  
even 1 fs is fairly good!

keep your eyes wide opened (if there's a mistype in the units in the  
book or paper) since
something that looks like water but with a bending constant of more  
than 300 kjmol&deg^-2
will never behave like water!


On 8 Oct 2008, at 11:04, nadler wrote:

> Well, I have to say that I am VERY glad that you and all the people
> around you are still up and healthy! :D
> I tried a time step of 0.05 fs and it works... But it is strange
> because in the paper they reported a time step of 0.75 fs (which I
> tested and it doesn't work as well) and the parameters I used where
> from Table 1, "Parameteres for water in ambient condition in this
> work". Therefore the numbers should be ok, according to the paper. So
> I changed the units to kjmol*rad^-2 and it worked too!! The
> fluctuations in the conserved quantity are small.
> Thanks Axel and Teo for guiding me to a working input :)
> Roger
> On 7 oct, 23:38, Laino Teodoro <teodor... at gmail.com> wrote:
>> Dear Roger,
>> do you have an idea of what 353.0  kjmol*deg^-2  mean?
>> if you are at the equilibrium position, and you compress or enlarge
>> the H-O-H angle of just one small irrelevant degree
>> the energy of your system would increase of ~ 85 kcal. This means
>> that thermal fluctuations of few degrees normally connected with the
>> bending H-O-H would be enough to break chemical bonds.
>> That's why I'm not wondered to hear from you that your water is
>> exploding!.. I'm not sure.. I had no time to test your input.. but  
>> the
>> result should be roughly like the famous Tsar bomb (if you're
>> curious:http://www.youtube.com/watch?v=FfoQsZa8F1c)
>> Since I drunk few minutes ago a fresh glass of water (kindly offered
>> from the Zurich water supply network), I can witness
>> that it was not like the plasma you call water! ;-)
>> So.. I would think a bit more on your units!
>> of course as Axel suggested, with a potential like this one.. maybe
>> an integration time step of 0.05-0.1 fs is enough.. not for
>> getting a proper conserved quantity.. but at least to tame the
>> integrators...
>> Teo
>>>       &BEND
>>>         ATOMS H O H
>>>         KIND HARMONIC
>>>         K [kjmol*deg^-2] 353.0
>>>         THETA0 [deg] 109.4712
>>>       &END BEND
> >

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