Right, I think I missed a point that whether SHAKE is used or not, TIP3P model is set as rigid
(since H-H bond exists there). So the HW-OW-HW force constant is just a placeholder for it.
HW-OW-HW has angle force constant as 37.95 kcal/(mol*Rad^2) in the SPC/Fw model,
which is close to the B3LYP/6-31G* level of theory calculated Hessian matrix predicted
force constant (which is 49.51 kcal/(mol*Rad^2)).
> On Apr 5, 2018, at 8:25 AM, David Cerutti <dscerutti.gmail.com> wrote:
>
> Look up the SPC-Fw paper by Yujie Wu. The angle constant, which is
> probably tuned to get bulk properties just like everything else, would be
> interdependent on the charges and vdW parameters. If you want to fit an
> angle parameter to reproduce specific quantum data, use mdgx and see what
> comes out. Another thing to point out for avid readers is that TIP3P and
> other rigid models have a bond between the hydrogens to enforce their
> geometry.
>
> On Wed, Apr 4, 2018 at 8:22 PM, pengfei li <ldsoar1990.gmail.com> wrote:
>
>> Hi Carlos and Dave,
>>
>> That is a good point. It seems it doesn’t matter when using SHAKE since it
>> is rigid.
>> I am just curious if SHAKE was not used, is 50.0 kcal/(mol*Rad^2) a better
>> choice
>> than 100.0 kcal/(mol*Rad^2)?
>>
>> Best regards,
>> Pengfei
>>
>>> On Apr 5, 2018, at 8:20 AM, David Cerutti <dscerutti.gmail.com> wrote:
>>>
>>> I'm in agreement with Carlos--the force constants for rigid water models
>>> are placeholders. For SPC-Fw, the angle constant does matter, but
>>> understand it's essentially making a fixed charge water model
>> polarizable,
>>> in a sense.
>>>
>>> On Wed, Apr 4, 2018 at 8:17 PM, Carlos Simmerling <
>>> carlos.simmerling.gmail.com> wrote:
>>>
>>>> Since the water is rigid and there is a bond between the H atoms, does
>> the
>>>> force constant matter?
>>>>
>>>> On Wed, Apr 4, 2018, 8:14 PM pengfei li <ldsoar1990.gmail.com> wrote:
>>>>
>>>>> Hi Everyone,
>>>>>
>>>>> Does anybody know how does the force constant for HW-OW-HW angle come
>>>>> from? I found it is 100.0 kcal/(mol*Rad^2) but 50.0 kcal/(mol*Rad^2)
>> is a
>>>>> more reasonable choice based on Hessian matrix of B3LYP/6-31G* level of
>>>>> calculations. However, I could not find a value in the original
>>>> TIP3P/TIP4P
>>>>> article (Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R.
>>>> W.;
>>>>> Klein, M. L. J. Chem. Phys. 1983, 79, 926-935.). So I am just curious
>>>> about
>>>>> what does the original value come from? Thanks!
>>>>>
>>>>> Best regards,
>>>>> Pengfei
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Received on Wed Apr 04 2018 - 17:30:06 PDT
