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From: |
Michael R Shirts <Michael.Shirts@...1657...> |

Date: |
Thu, 20 Jul 2017 19:54:23 +0000 |

Following up on Steve Plimpton’s pointing out that I was assuming ps as the real time unit, rather than fs, I tried to rerun to make the simulations more comparable. However this made no difference: it was statistically identical, though it’s numerically even clearer that there is an issue, since the number of uncorrelated samples increased with the 1000x longer timestep. I changed the time for temperature and pressure pcoupling to 400 and 2000 time steps to match the GROMACS simulations more precisely. I’m attaching the new files. Note that I independently, added ‘mtk yes’ to the previous simulations (i.e. changed “fix id1 all npt iso 797 797 5.0 temp 44 44 5.0”, to “fix id1 all npt mtk yes iso 797 797 5.0 temp 44 44 5.0”), which resulted in literally no change – it was identically numerically out to at least 50K steps. Also, I verified that the average pressure estimator output by LAMMPS is statistically correct (for 797 simulation, pressure is 796.6 +/- 0.9 . So the average pressure is right (I didn’t expect that to be wrong, so good ☺. Data below: BEFORE (time step really small): T P <V (A^3)> dV/<V> 44 797 36833.043 0.002410 44 897 36751.109 0.002408 compressibility from finite difference = 2.2565e-05 atm^-1 compressibility from fluctuation (at 797 atm) = 3.5237e-05 atm^-1 compressibility from fluctuation (at 897 atm) = 3.5108e-05 atm^-1 compressibility from fluctuation (average) = 3.5173e-05 atm^-1 Validation of NPT ensemble looking at slope of log P1(V)/P2(V) --------------------------------------------- Estimated slope vs. True slope --------------------------------------------- -9.768420 +/- 0.462170 | -17.006006 --------------------------------------------- (That's 15.66 quantiles from true slope=-17.006006) --------------------------------------------- True dP = 100.000, Eff. dP = 57.441+/-2.718 --------------------------------------------- NOW (time step 5 fs) T P <V (A^3)> dV/<V> 44 797 36833.630 0.002426 44 897 36749.948 0.002431 compressibility from finite difference = 2.3046 atm^-1 compressibility from fluctuation (at 797 atm) = 3.5693e-05 atm^-1 compressibility from fluctuation (at 897 atm) = 3.5772e-05 atm^-1 compressibility from fluctuation (average) = 3.5733e-05 atm^-1 --------------------------------------------- Estimated slope vs. True slope --------------------------------------------- -10.557018 +/- 0.301752 | -17.006006 --------------------------------------------- (That's 21.37 quantiles from true slope=-17.006006, FYI.) --------------------------------------------- True dP = 100.000, Eff. dP = 62.078+/-1.774 --------------------------------------------- ~~~~~~~~~~~~~~~~ Michael Shirts Associate Professor michael.shirts@...780... http://www.colorado.edu/lab/shirtsgroup/ Phone: (303) 735-7860 Office: JSCBB C123 Department of Chemical and Biological Engineering University of Colorado Boulder On 7/19/17, 1:17 PM, "Michael R Shirts" <Michael.Shirts@...1657...> wrote: Hi, all- I’m using MD to simulate FCC Lennard-Jones. I am not getting consistent values between compressibility determined by finite difference and by fluctuation formulas. I tried it in reduced units, was getting inconsistent results, and switched to real units to make it easier to compare numbers. I’ve also compared to GROMACS, where I get much better consistency. This appears to be an underlying issue with LAMMPS not getting the proper Boltzmann distribution for the NPT ensemble. I wonder if there is some subtle setting I’m getting wrong? Some info: Formula for finite difference used = - <Vhigh>-<Vhlow>/(((Phigh-Plow) (0.5(<Vhigh> + vlow>))) Formula for finite difference by fluctuation: std(V)^2/kBT*<V>, kB = 0.1380 bar/nm^3 / K. LAMMPS: T P <V (A^3)> dV/<V> 44 797 36833.043 0.002410 44 897 36751.109 0.002408 compressibility from finite difference = 2.2565e-05 atm^-1 compressibility from fluctuation (at 797 atm) = 3.5237e-05 atm^-1 compressibility from fluctuation (at 897 atm) = 3.5108e-05 atm^-1 compressibility from fluctuation (average) = 3.5173e-05 atm^-1 GROMACS: T P <V (nm^3)> dV/<V> 44 797 37.33091 0.002465 44 897 37.19381 0.002427 compressibility from finite difference = 3.6792e-05 bar^-1 compressibility from fluctuation (at 797 bar) = 3.7367e-05 bar^-1 compressibility from fluctuation (at 897 bar) = 3.6073e-05 bar^-1 compressibility from fluctuation (average) = 3.6720e-05 bar^-1 I expect the fantastic agreement of GROMACS to be somewhat a statistical fluke, but it is consistent. I’ve also run ‘checkensemble’ (https://github.com/shirtsgroup/checkensemble) on the volume fluctuations. In this case, I got for the maximum likelihood analysis of the slope of log P_1(V)/P_2(V): GROMACS: --------------------------------------------- Estimated slope vs. True slope --------------------------------------------- -15.963630 +/- 0.345655 | -17.006006 --------------------------------------------- (That's 3.02 quantiles from true slope=-17.006006) --------------------------------------------- True dP = 100.000, Eff. dP = 93.871+/-2.033 --------------------------------------------- And for LAMMPS: --------------------------------------------- Estimated slope vs. True slope --------------------------------------------- -9.768420 +/- 0.462170 | -17.006006 --------------------------------------------- (That's 15.66 quantiles from true slope=-17.006006) --------------------------------------------- True dP = 100.000, Eff. dP = 57.441+/-2.718 --------------------------------------------- Which seems problematic (GROMACS is a bit off, but much less so). I’ve attached the lammps input files. To analyze them, I threw out the first 50000 steps for equilibration. I’ve attached the GROMACS mdp files (though not the coordinates and topologies because of size. There are a few differences between the inputs, though they shouldn't affect the comparison I’m doing. I accidentally didn’t convert from bar to atm when going between GROMACS and LAMMPS runs, but did in the analysis. I don’t expect the average volumes to be exactly the same between GROMACS and LAMMPS because of the way the cutoffs are handled, but they should be internally consistent within the program. The dimensions of the box are not quite the same between the two programs for various reasons, but they are both roughly square, with 768 atoms. There are some difference in tau_t and tau_p, though they are all differences that shouldn’t really matter. Parameters should be the same – though again, it shouldn’t matter since we’re looking at internal consistency. Here, GROMACS uses Martyna-Tuckerman-Tobias-Klein equations, and did not use mtk yes with lammps, though I would think the 1/V term should be negligible here (though I will test it). Any insight would be useful! Best, ~~~~~~~~~~~~~~~~ Michael Shirts Associate Professor michael.shirts@...780... http://www.colorado.edu/lab/shirtsgroup/ Phone: (303) 735-7860 Office: JSCBB C123 Department of Chemical and Biological Engineering University of Colorado Boulder

**Attachment:
in.real.44_897.LJ**

**Attachment:
in.real.44_797.LJ**

**Follow-Ups**:**Re: [lammps-users] Issue with pressure control not giving the correct fluctuation properties with Lennard-Jones solids***From:*Michael R Shirts <Michael.Shirts@...1657...>

**References**:**[lammps-users] Issue with pressure control not giving the correct fluctuation properties with Lennard-Jones solids***From:*Michael R Shirts <Michael.Shirts@...1657...>

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