[gmx-developers] Scaling velocities in Velocity Verlet

[David van der Spoel]

On 12/4/11 4:35 AM, Shirts, Michael (mrs5pt) wrote:
> Hmm.  Which version/branch is this?
This is the qhop branch, based on the 4.5 branch.


>
> Best,
> ~~~~~~~~~~~~
> Michael Shirts
> Assistant Professor
> Department of Chemical Engineering
> University of Virginia
> michael.shirts at virginia.edu
> (434)-243-1821
>
>
>> From: David van der Spoel<spoel at xray.bmc.uu.se>
>> Reply-To: Discussion list for GROMACS development<gmx-developers at gromacs.org>
>> Date: Sat, 3 Dec 2011 22:10:57 +0100
>> To: Discussion list for GROMACS development<gmx-developers at gromacs.org>
>> Subject: Re: [gmx-developers] Scaling velocities in Velocity Verlet
>>
>> On 2011-12-03 21:37, Shirts, Michael (mrs5pt) wrote:
>>> Hi, all-
>>>
>>>> The problem is that the change in Ekin comes one step too late. I tried
>>>> to deduce the algorithm from the code (4.5 code base) and deduced the
>>>> following MD algorithm:
>>>
>>> It's rather difficult to follow currently because of the need to bookkeep
>>> both md and md-vv + plus correct pressure control w/constraints. I have high
>>> hopes for improving this for 5.0.
>>>
>>>> v(t) = v(t-dt/2) + a(t) dt/2
>>>>
>>>> v'(t) = lambda v(t)<======= my addition
>>>>
>>>> v(t+dt/2) = v'(t) + a(t) dt/2
>>>>
>>>> r(t+dt) = r(t) + v(t+dt/2) dt
>>>>
>>>> However, the Ekin that is printed is the uncorrected one even though I
>>>> scale both ekind->ekin and ekind->tcstat[x].ekinf .
>>>>
>>>> Is the algorithm correct as I write it above? Is the Ekin(t) already
>>>> stored at the beginning of the time step?
>>>
>>> If you add your modification as an option to update_tcouple, and operate on
>>> ekind directly (not the velocities), in the same way that berendsen_tcouple
>>> does, then the bookkeeping should take care of the velocity rescaling
>>> appropriately.
>> Close, but no cigar.
>> Here's the output from g_energy:
>>
>> @ s0 legend "Potential"
>> @ s1 legend "Kinetic En."
>> @ s2 legend "Total Energy"
>>       0.029600  -35405.431557  6460.241804  -28945.189753
>>       0.029800  -35407.596247  6462.418642  -28945.177606
>>       0.030000  -35327.293038  6466.347370  -28860.945667
>>       0.030200  -35330.858001  6387.271571  -28943.586430
>>       0.030400  -35334.701690  6391.114750  -28943.586940
>>
>> The jump in Potential is at t = 0.03, in Kinetic it is at t = 0.302. As
>> you see the Total energy has a peak at t = 0.03, but it does come back
>> almost to the right level at the next step. This all makes me think I
>> should be scaling the previous velocities, hence my questions about the
>> algorithm.
>>
>>
>>
>>>
>>> If this _doesn't_ work, then there's some tweaks that got lost in the
>>> different versions that are floating around, and I can either put the
>>> changes into 4.6 alone, or just add them in with the 4.6 merge.
>>>
>>> Best,
>>> ~~~~~~~~~~~~
>>> Michael Shirts
>>> Assistant Professor
>>> Department of Chemical Engineering
>>> University of Virginia
>>> michael.shirts at virginia.edu
>>> (434)-243-1821
>>>
>>>
>>>
>>>> v(t) = v(t-dt/2) + a(t) dt/2
>>>>
>>>> v'(t) = lambda v(t)<======= my addition
>>>>
>>>> v(t+dt/2) = v'(t) + a(t) dt/2
>>>>
>>>> r(t+dt) = r(t) + v(t+dt/2) dt
>>>>
>>>> However, the Ekin that is printed is the uncorrected one even though I
>>>> scale both ekind->ekin and ekind->tcstat[x].ekinf .
>>>>
>>>> Is the algorithm correct as I write it above? Is the Ekin(t) already
>>>> stored at the beginning of the time step?
>>>>
>>>>
>>>> --
>>>> David van der Spoel, Ph.D., Professor of Biology
>>>> Dept. of Cell&   Molec. Biol., Uppsala University.
>>>> Box 596, 75124 Uppsala, Sweden. Phone: +46184714205.
>>>> spoel at xray.bmc.uu.se    http://folding.bmc.uu.se
>>>> --
>>>> gmx-developers mailing list
>>>> gmx-developers at gromacs.org
>>>> http://lists.gromacs.org/mailman/listinfo/gmx-developers
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>>>
>>
>>
>> --
>> David van der Spoel, Ph.D., Professor of Biology
>> Dept. of Cell&  Molec. Biol., Uppsala University.
>> Box 596, 75124 Uppsala, Sweden. Phone: +46184714205.
>> spoel at xray.bmc.uu.se    http://folding.bmc.uu.se
>> --
>> gmx-developers mailing list
>> gmx-developers at gromacs.org
>> http://lists.gromacs.org/mailman/listinfo/gmx-developers
>> Please don't post (un)subscribe requests to the list. Use the
>> www interface or send it to gmx-developers-request at gromacs.org.
>


-- 
David.
________________________________________________________________________
David van der Spoel, PhD, Professor of Biology
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,  	75124 Uppsala, Sweden
phone:	46 18 471 4205		fax: 46 18 511 755
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se
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