Re: [AMBER] AMBER 14 DPFP single energy calculations inconsistent

From: R.G. Mantell <>
Date: Wed, 28 Jan 2015 18:27:41 +0000

Hi Ross,

The EGB energies in my previous email are from my original input but
using your suggestion of 'imin=0, nstlim=1, ntpr=1' in

I have some structures which are slightly lower in RMS force here:
They were generated using our CUDA L-BFGS minimiser interfaced with
AMBER 12 DPDP and I see the same problems when the structures are put
into pmemd.cuda. Unfortunately I can't get any structures which have a
very low RMS force as the inconsistent energies are confusing the line
search in the minimiser, but I've managed to get it a bit lower than the
original structure. Still waiting on the CPU minimisation...

I don't know whether it happens with PME simulations as we only have GB
interfaced with our code.



On 2015-01-27 18:54, Ross Walker wrote:
> Hi Rosemary,
> Okay, this definitely looks like a bug - although a weird one. Can you
> send me the input files you used for the EGB energies you list below
> (the structure with a low RMS force?) and I'll test this.
> One quick question - does this only happen with GB simulations - or
> have you seen such behavior with PME simulations as well?
> All the best
> Ross
>> On Jan 27, 2015, at 10:13 AM, Rosemary Mantell <>
>> wrote:
>> Hi Ross,
>> I should probably mention that I first saw this problem when using the
>> AMBER 12 DPDP model, so it's not just a problem with fixed precision.
>> I've set a longer CPU minimisation running today as you suggested,
>> though it will be a little while until it finishes. I will let you
>> know
>> what I find when it's done. However, I also have an L-BFGS minimiser
>> written in CUDA that I have interfaced with the AMBER 12 DPDP
>> potential
>> and I have been using this to run minimisations with this system.
>> Although the minimisations don't converge properly (the linesearch in
>> the minimiser is not tolerant of the fluctuating energies that are
>> being
>> produced), I was able to generate some structures with a much lower
>> RMS
>> force and put these back into pmemd.cuda. I am still seeing the same
>> problem with DPFP and not with SPFP for a variety of different
>> structures.
>> I also tried 'imin=0, nstlim=1, ntpr=1' and the EGB energies I got for
>> 10 tests with DPFP are: -119767.0113, -119763.2412, -119764.3177,
>> -119764.4183, -119763.3771, -119765.8321, -119765.3539, -119764.3328,
>> -119764.1440, -119764.9855.
>> Thanks,
>> Rosie
>> On 26/01/2015 15:48, Ross Walker wrote:
>>> Hi Rosie,
>>> This does indeed look concerning. Although is not surprising if your
>>> structure is highly strained. The fixed precision model is such that
>>> if energies or forces are too large they will overflow the fixed
>>> precision accumulators. This should never happen during MD since the
>>> forces would be so large as to cause the system to explode. But it
>>> can happen in minimization - but given minimization is designed just
>>> to clean up highly strained structures it should not be a concern.
>>> The first thing we should do though is establish if this is the case
>>> here or if this is a more deeply rooted bug.
>>> Can you first run a few thousand steps of minimization of your
>>> structure using the CPU and then from the restart files you get from
>>> that repeat your tests (just pick a single GPU model and CUDA version
>>> as that should not be relevant unless the GPU is faulty but that's
>>> unlikely given what you describe) - try it 10 times or so with SPFP
>>> and DPFP and see what you get. This will give us an idea of where to
>>> start looking.
>>> Could you also try, instead of imin=1 setting:
>>> imin=0, nstlim=1, ntpr=1 and see what you get reported there for the
>>> energies. This does the same calculation but throuhg the MD routines
>>> rather than the minimization routines.
>>> When I get a chance later today I'll also try it on my own machine
>>> with the input you provided.
>>> All the best
>>> Ross
>>>> On Jan 26, 2015, at 7:34 AM, R.G. Mantell <> wrote:
>>>> I'm not doing a full minimisation. I am using imin = 1, maxcyc = 0,
>>>> ncyc
>>>> = 0, so would hope to get the same energy if I ran this same
>>>> calculation using DPFP several times. Running five times I get: EGB
>>>> =-119080.5069, EGB = -119072.8449, EGB = -119079.8208, EGB =
>>>> -119076.1230, EGB = -119073.7929
>>>> If I do this same test with another system, I get the same EGB
>>>> energy
>>>> every time.
>>>> Thanks,
>>>> Rosie
>>>> On 2015-01-26 15:09, David A Case wrote:
>>>>> On Mon, Jan 26, 2015, R.G. Mantell wrote:
>>>>>> I am having some problems with pmemd.cuda_DPFP in AMBER 14 and
>>>>>> also
>>>>>> seeing the same problems in AMBER 12 with DPDP and SPDP precision
>>>>>> models. I have some input for which a single energy calculation
>>>>>> does
>>>>>> not
>>>>>> yield the same energy each time I run it. Looking at min.out, it
>>>>>> seems
>>>>>> that it is the EGB component which gives a different value each
>>>>>> time.
>>>>>> This does not occur when using SPFP or the CPU version of AMBER. I
>>>>>> do
>>>>>> not see this problem when using input for other systems. I have
>>>>>> tried
>>>>>> the calculation on a Tesla K20m GPU and a GeForce GTX TITAN Black
>>>>>> GPU
>>>>>> using several different versions of the CUDA toolkit. I see the
>>>>>> same
>>>>>> problem with both igb=1 and igb=2. The input which causes the
>>>>>> problem
>>>>>> can be found here:
>>>>> Can you say how different the values are on each run? What you
>>>>> describe is
>>>>> exactly what should be expected: parallel runs (and all GPU runs
>>>>> are
>>>>> highly
>>>>> parallel) with DPDP or SPDP are not deterministic, whereas Amber's
>>>>> SPFP
>>>>> is.
>>>>> On the other hand, if you are seeing significant differences
>>>>> between
>>>>> runs for
>>>>> DPDP, that might indicate a bug that needs to be examined.
>>>>> ...thx...dac
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Received on Wed Jan 28 2015 - 10:30:05 PST
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