Calculating surface tension in a water/vapour interface

LAMMPS LAMMPS General Discussion
1

Dear all,

I am attempting to run a coarse grained liquid/vapour simulation to find the effects of varying different paramaters on lammps on the surface tension. I have attached my script below:

------------ Create simulation box ----------------

units real
atom_style sphere
boundary p s p

lattice sc 3.1
region box block -10 10 -20 20 -10 10
create_box 1 box
region atoms block -2.5 2.5 -2.5 2.5 -2.5 2.5
region puddle block -10 10 -2.5 2.5 -10 10

-------------- Create Atom type 1 ----------------------

lattice sc 3.1
create_atoms 1 region puddle ratio 1.0 1234

------------------ Setup potential (Force field) -----------------

pair_style lj/cut 3.5
pair_coeff 1 1 10.0 2.3

gives good surface tension with values of 10.0 and 2.35

#----------------- Units --------------------------
variable A_in_m equal 1e-10 # Angstrom in meter
variable atm_in_Pa equal 101325 # note: 1 Pa = 1 N/m^2
variable N_in_mN equal 1e3 # Newton in milliNewton
variable convFac equal {A_in_m}*{atm_in_Pa}*${N_in_mN}

----------------- Paramaeteres ---------------------

thermo 2000
timestep 0.01
neighbor 3.5 bin
neigh_modify every 1 delay 0 check yes
group cold type 1

variable Ne equal 10
variable Nr equal 500
#variable Nf equal 100

------------------ Give velocity and fix commands ------------------

velocity all create 230 1234 # dist gaussian rot yes
fix 1 all nve/sphere
fix 2 all momentum 1 linear 1 1 1
fix temp all langevin 230 230 1000 1234 omega yes zero yes

fix 4 all npt temp 273.0 273.0 100 iso 1.0 1.0 1000 # works well at temp 100 press 1000

fix 3 all nvt temp 273.0 273.0 100.0

fix 4 cold gravity 100.0 vector 0 1 0

fix PAve all ave/time {Ne} {Nr} 1000 c_totT

fix totT all ave/time {Ne} {Nr} 1000 c_rotT

---------------- Density ---------------------

set group cold density 1.0

------- Pressure, temperature, tensor and surface tension ---------------

compute T all temp
compute P all pressure T

variable xPress equal c_thermo_press[1]
variable yPress equal c_thermo_press[2]
variable zPress equal c_thermo_press[3]

variable st equal 0.5lz(v_zPress-0.5*(v_xPress+v_yPress))*${convFac}

fix st all ave/time 1 2000 2000 v_st
#fix st all ave/time 10 100 100 v_st
#fix 3 st all ave/time c_myTemp c_thermo_temp file temp.profile

#------------------ Output --------------------
thermo_style custom step f_st temp press etotal
thermo_modify lost warn
dump trj all atom 2000 testcgoneatom.lammpstrj
run 2000000

However, upon running my simulation, the surface tension i get always seems to be negative. A negative number between zero and negative 30.

This is what the interface looks like on onvito

image

I am very unsure why i am getting a negative surface tension and would really love some help from anyone.

Kind regards,

Zakaria

2

Just to clarify, the lines in bold in the input script are not being used for the simulation. They have a #before them which doesn’t show up when I paste my script here.

3

to get correct typesetting of input files, you should bracket them in triple backquote (```) sections (similar to python docstrings). Like this:

# 3d Lennard-Jones melt

units           lj
atom_style      atomic

lattice         fcc 0.8442
region          box block 0 10 0 10 0 10
create_box      1 box
create_atoms    1 box
mass            1 1.0

velocity        all create 3.0 87287 loop geom

pair_style      lj/cut 2.5
pair_coeff      1 1 1.0 1.0 2.5

neighbor        0.3 bin
neigh_modify    every 20 delay 0 check no

fix             1 all nve

#dump           id all atom 50 dump.melt
#dump           2 all image 25 image.*.jpg type type &
#               axes yes 0.8 0.02 view 60 -30
#dump_modify    2 pad 3

#dump           3 all movie 25 movie.mpg type type &
#               axes yes 0.8 0.02 view 60 -30
#dump_modify    3 pad 3

thermo          50
run             250
4

Ah yes I see. Thank you my friend! I will keep this in mind for any later posts I make :slight_smile:

5

Unless you edit your existing post, it is not likely to get constructive suggestions in this topic, since it is not possible to properly assess it or reproduce it.

6

Yes, i am going to delete this post, I noticed some errors in my code that my supervisor told me about. Thank you for the advice akohlmey, take care my friend.

7

Hmm it seems as though i cannot delete nor edit the post. But thank you for telling me how to properly post on this forum, i will make a better post soon.

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