[gmx-users] 'constraints' directive and water

[Alex]

My idea about SETTLE probably comes from misunderstanding of the fact 
that I do not have FLEXIBLE defined anywhere and the following excerpt 
from tip4.itp:

#ifndef FLEXIBLE
[ settles ]
; OW    funct   doh        dhh
1       1       0.09572    0.15139
#else
[ bonds ]
; i    j    funct    length    force.c.
1    2    1    0.09572    502416.0 0.09572    502416.0
1    3    1    0.09572    502416.0 0.09572    502416.0


On 3/27/2018 1:18 PM, Alex wrote:
> That was always my default impression, but from looking into e.g. 
> tip4p.itp i somehow got the idea that water _is_ affected by 
> 'constraints'.
>
> Yes, whenever I have hydrogens in the system (aside from water), I 
> always set 'h-bonds', which, if you recall my old 'meh' regarding 
> constraints, suggests that I have accepted your view for computational 
> efficiency. :)  This certainly answers my question, thank you.
>
> Alex
>
>
> On 3/27/2018 1:11 PM, Justin Lemkul wrote:
>>
>>
>> On 3/27/18 3:10 PM, Alex wrote:
>>> Hi all,
>>>
>>> Some of my simulations run with a time step of 2 fs and 
>>> 'constraints' set to 'none'. Everything runs perfectly fine, but 
>>> with such a time step, do you usually set constraints to 'h-bonds'? 
>>> I know I should be looking at literature for this, but just wanted 
>>> to get a basic idea on whether people do this.
>>>
>>
>> Normally constraining bonds to H is required to get stable 
>> integration with dt = 2 fs. Water is a different matter; it is held 
>> rigid with SETTLE and is not under the influence of the "constraints" 
>> keyword.
>>
>> -Justin
>>
>