Density of methanol

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1

To lammps users,

I am testing the density of methanol using a given example of LAMMPS software (~/LAMMPS/Examples/dreading/)

It is known that the density of methanol is ~0.792 g/cm3.

The initial density of methanol example was 0.457 g/cm3 but the density is lower, showing the enlarged unitcell.

How can I get reasonable vlaue (~0.792 g/cm3)?

results##

Step Temp TotEng PotEng KinEng Lx Ly Lz Press Density
0 298.15 12308.081 3094.6323 9213.4485 59.99067 57.38448 58.40913 1781.0418 0.4572555
1000 290.58157 19927.153 10947.585 8979.5683 70.096033 67.050833 68.248084 -92.74926 0.28663535
2000 305.64025 21030.387 11585.475 9444.9127 81.307743 77.77547 79.164219 60.844666 0.1836599
3000 302.94026 21826.71 12465.233 9361.4774 93.059138 89.016346 90.605811 97.386573 0.12249913

input script

units real
atom_style full
boundary p p p
dielectric 1
special_bonds lj/coul 0.0 0.0 1.0

pair_style hybrid/overlay hbond/dreiding/lj 2 6 6.5 90 lj/cut/coul/long 8.50000 11.5
bond_style harmonic
angle_style harmonic
dihedral_style harmonic
improper_style none
kspace_style pppm 0.001

read_data data.dreiding

replicate 3 3 3

pair_coeff 1 1 lj/cut/coul/long 0.015200000256300 2.846421344984478
pair_coeff 1 2 lj/cut/coul/long 0.001232882795416 2.846421344984478
pair_coeff 1 3 lj/cut/coul/long 0.038019995160237 3.159705878878677
pair_coeff 1 4 lj/cut/coul/long 0.038139744011598 2.939787518071103
pair_coeff 2 2 lj/cut/coul/long 9.99999974737875e-05 2.846421344984478
pair_coeff 2 3 lj/cut/coul/long 0.003083828758188 3.159705878878677
pair_coeff 2 4 lj/cut/coul/long 0.003093541672406 2.939787518071103
pair_coeff 3 3 lj/cut/coul/long 0.095100000500679 3.472990412772877
pair_coeff 3 4 lj/cut/coul/long 0.095399530150179 3.253072051965302
pair_coeff 4 4 lj/cut/coul/long 0.095700003206730 3.033153691157727
pair_coeff 4 4 hbond/dreiding/lj 2 i 0.4000E+01 2.750000000000000 4
pair_modify mix arithmetic
neighbor 2.0 multi
neigh_modify every 2 delay 4 check yes
variable input index in.ch3oh.box.dreiding
variable sname index ch3oh.box.dreiding

compute hb all pair hbond/dreiding/lj
variable C_hbond equal c_hb[1] #number hbonds
variable E_hbond equal c_hb[2] #hbond energy
#thermo_style custom etotal ke temp pe ebond eangle edihed eimp evdwl ecoul elong v_E_hbond v_C_hbond press vol
#thermo_modify line multi format float %14.6f

Output

thermo 1000
thermo_style custom step temp etotal pe ke lx ly lz press density

dump myOvito all custom 1000 dump.ovito.* id type xs ys zs

################### NPT
timestep 1
velocity all create 298.15 4928459 rot yes mom yes dist gaussian
#fix ensemble_press all press/berendsen iso 1.0 1.0 1000.0
fix fxnpt all npt temp 300.0 300.0 100.0 iso 1.0 1.0 100.0
run 100000
unfix fxnpt
#unfix ensemble_press
reset_timestep 0

2

To lammps users,

I am testing the density of methanol using a given example of LAMMPS
software (~/LAMMPS/Examples/dreading/)

It is known that the density of methanol is ~0.792 g/cm3.

The initial density of methanol example was 0.457 g/cm3 but the density is
lower, showing the enlarged unitcell.

How can I get reasonable vlaue (~0.792 g/cm3)?

​different parameterizations in classical MD will reproduce experimentally
data differently well.
there is no compound and not set of force field parameters that reproduces
every experimental property exactly. parameterizations are always a
compromise.
in simple empirical potentials, the parameterization is often limited as to
what themodynamic states can be reproduced well.

to find a parameterization that reproduces methanol more to your liking,
you will have to study the published literature. folks have been studying
methanol (and methanol mixtures) for as long as i am doing simulations,
i.e. over 20 years. so there should be plenty of literature to check out.

​axel.​