[lammps-users] 2D system with charged particles

Hello, everyone,

I am new to use LAMMPS. After several trials to write a input script, I can finally run a
simulation. However, I have some concerns about the result from the log file. First, I would like
to ask for help to check my input script is proper to my system. My system is a two dimensional
canonical ensemble with two different charg spherical particles (course-grained model) in both size
and charge. The following is my input script file:

#initialization section

units lj #default
dimension 3
newton on #default
boundary p p f
atom_style charge

# atom definition section by reading data files
read_restart restart.file

# LJ potentials #epslong sigma cutoff

pair_style lj/cut/coul/long 2.5 6.57353
pair_coeff 1 1 1.0 5.0 5.61230
pair_coeff 2 2 1.0 1.0 1.12246
pair_coeff 1 2 1.0 5.5 6.57353

kspace_style pppm 0.0001
kspace_modify slab 3.0

#setting simulation parameters

neighbor 0.3 bin
neigh_modify delay 5

# define groups

group big type 1
group small type 2
compute big_temp big temp
compute small_temp small temp

# initial velocities

velocity all create 1.0 482748 #generate temperature by random
seed

# Run

fix 1 all nvt 1.0 1.0 100
fix 2 all enforce2d
timestep 0.003
thermo 500
restart 10000 restart.*.run
dump 1 all custom 50 dump.*.xy tag type x y q
run 100000

The following is part of the log file:

PPPM initialization ...
  G vector = 0.458211
  grid = 150 150 8
  RMS precision = 6.64569e-05
  brick FFT buffer size/proc = 240250 180000 144150
Memory usage per processor = 27.891 Mbytes
Step Temp E_pair E_mol TotEng Press
       8 0.66238482 61.3859 0 62.378925 1.5982966
     500 3.1237375 57.531757 0 62.21476 2.5530711
    1000 7.1666455 51.374965 0 62.118961 2.8867492
    1500 10.690227 46.034204 0 62.060636 3.1953576
    2000 14.143277 40.780454 0 61.983583 3.3592176
    2500 17.532012 35.395325 0 61.678733 3.501395
    3000 19.88523 31.316189 0 61.127463 3.5465346
    3500 21.134675 28.566923 0 60.251323 3.7681761
    4000 22.599229 25.134924 0 59.014935 3.6816382
    4500 23.400345 22.272707 0 57.353725 3.7382589
    5000 23.619302 19.915203 0 55.324474 3.6222621
    5500 23.767282 17.278699 0 52.909815 3.3814601
    6000 23.297723 15.194923 0 50.122093 3.2909656
    6500 22.720556 12.984601 0 47.046501 3.0173705
    7000 21.128461 12.129668 0 43.804752 3.0182893
    7500 19.9004 10.677082 0 40.511099 2.7053707
    8000 18.340354 9.6737243 0 37.168972 2.5714761
    8500 16.842243 8.6632206 0 33.91255 2.3929294
    9000 15.24571 7.9242938 0 30.780154 2.2505554
    9500 13.650693 7.3240822 0 27.788747 2.0842213
   10000 12.277207 6.6090083 0 25.014588 1.9009692
   10500 10.78831 6.2954587 0 22.468933 1.7218264
   11000 9.5798016 5.7484051 0 20.110124 1.5579472
   11500 8.4212013 5.3996393 0 18.024424 1.3881848
   12000 7.3821743 5.0419769 0 16.109087 1.2461509
   12500 6.4067154 4.8285684 0 14.433303 1.1784767
   13000 5.6209136 4.4928053 0 12.919492 1.0485531
   13500 4.8431509 4.3511836 0 11.611874 0.98722131
   14000 4.2573911 4.0682514 0 10.45079 0.86245042
   14500 3.7617531 3.782121 0 9.4216159 0.78489859
   15000 3.3404522 3.4893526 0 8.4972472 0.6998342
   15500 2.9219959 3.321131 0 7.7016898 0.65552081
   16000 2.5441753 3.1760523 0 6.9901951 0.59510415
   16500 2.205009 3.037887 0 6.343563 0.57115684
   17000 2.0073448 2.7848967 0 5.7942412 0.49320819
   17500 1.8099599 2.5502294 0 5.2636611 0.46941982
   18000 1.6218368 2.3661631 0 4.7975667 0.42865997
   18500 1.4342993 2.2266904 0 4.3769441 0.42377277
   19000 1.2858617 2.0578749 0 3.9855958 0.39709195
   19500 1.1711925 1.8845381 0 3.6403509 0.36456088
   20000 1.0320959 1.8219582 0 3.3692419 0.34361724
   20500 0.97473772 1.621068 0 3.0823623 0.31173093
   21000 0.86513268 1.5134496 0 2.8104276 0.31893887
   21500 0.80022487 1.3798893 0 2.5795598 0.30419827
   22000 0.72578686 1.2723411 0 2.3604165 0.28560916
   22500 0.66622745 1.1537458 0 2.1525318 0.27106142
   23000 0.59296496 1.0840553 0 1.9730086 0.26757995
   23500 0.55329211 0.95591142 0 1.7853885 0.25554362
   24000 0.50735903 0.86573453 0 1.6263503 0.2516756
   24500 0.48070619 0.74417691 0 1.4648356 0.23914841
   25000 0.43335246 0.68343112 0 1.3330987 0.24110979
   25500 0.41341912 0.58299112 0 1.2027753 0.22060096
   26000 0.37744499 0.49507027 0 1.0609232 0.2215297
   26500 0.35235453 0.42130871 0 0.94954688 0.2149189
   27000 0.33017461 0.33353866 0 0.82852544 0.2067608
   27500 0.30598966 0.25970813 0 0.71843763 0.21097609
   28000 0.2873377 0.18946309 0 0.62023019 0.21071721
   28500 0.27460484 0.12547343 0 0.53715184 0.20049411
   29000 0.25155971 0.081238512 0 0.45836845 0.19899124
   29500 0.23182719 0.014105021 0 0.36165261 0.19424649
       ......

   97000 0.00037717509 -0.95519449 0 -0.95462904 0.15045272
   97500 0.0003836475 -0.95460281 0 -0.95402766 0.15032854
   98000 0.00037803279 -0.95738077 0 -0.95681403 0.15014553
   98500 0.00036874473 -0.95626278 0 -0.95570997 0.14993844
   99000 0.00037817216 -0.95306 0 -0.95249306 0.14985159
   99500 0.00041169825 -0.9537309 0 -0.95311369 0.14978556
  100000 0.00042346268 -0.95665005 0 -0.95601521 0.14985195
  100008 0.00042322864 -0.95665081 0 -0.95601632 0.14985362
    
The second column above is temperature. I have set the initial and final temperature to be 1 (LJ
unit, so reduced temperature). I am confused by the result. Could someone to help me figure out why
the temperaure is 0.00042322864 at the last step? Is there any mistake in my input script?

In my system, the two particles are asymmetric in size. I would like to use
neighbor 0.3 multi
but the program complains about it. It seems I have to use
neighbor 0.3 bin

My system is 2D. It seems I can't use "dimension 2" because I can't use "dimension 2" and "boundary
p p f" at the simutaneously. "pppm" is not compatible with "dimension 2" even I use
"kspace_modify slab 3.0". In a 2D system with charge particles, I have to use "dimension 3" in
order to use "pppm", is it correct?

Thank you inadvance and have a nice day!

Dongsheng

PPPM and Ewald only work with 3d simulations. I would do
a 3d periodic system first and see if you get reasonable long-range
energies and stability. Then make it non-periodic in z and see if
it is still stable.

Steve