# Fluctuation in calculating energy and specific heat

Dear LAMMPS users,

I am working on “Thermal properties of Self-Assembly monolayers at the liquid-solid interface” and want to calculate adsorption energy, enthalpy and specific heat with NVT ensembles using LAMMPS. But high fluctuations occurs in the temperature (± 30) and energy of the system during the simulations.

Can someone please give me suggestions how to minimize these fluctuations?

The input script for adsorption energy is

units real
atom_style charge
variable t equal 278
variable dt equal 0.01
variable system string combined

pair_style reax/c NULL
pair_coeff * * ./ffield.reax.cho C H O

velocity all create \$t 87287

neighbor 1.5 bin
neigh_modify once yes

fix 1 all box/relax iso 0.0 vmax 0.001
fix 2 all qeq/reax 1 0.0 10.0 1e-6 param.qeq
fix 3 all nvt temp \$t \$t 0.03

timestep \${dt}
thermo 100

run 500000

Kind Regards

Saeed

Dear LAMMPS users,

I am working on "Thermal properties of Self-Assembly monolayers at the
liquid-solid interface" and want to calculate adsorption energy, enthalpy
and specific heat with NVT ensembles using LAMMPS. But high fluctuations
occurs in the temperature (+- 30) and energy of the system during the
simulations.

...and what version of LAMMPS are you using? and on what platform are
you running? the mailing list guidelines state, that those are crucial
pieces of information.

Can someone please give me suggestions how to minimize these fluctuations?

some fluctuations are to be expected and depend on the system size.
there are multiple discussions on this subject in the mailing list
archives, and it should also be explained in any good text book on MD,
that for a system in equilibrium, the average over time is equivalent
to the average over the system.

The input script for adsorption energy is

now, there are multiple problems with this script. please see the

units real
atom_style charge
variable t equal 278
variable dt equal 0.01
variable system string combined

pair_style reax/c NULL
pair_coeff * * ./ffield.reax.cho C H O

as it says in the header of the ffield.reax.cho file, this is a force
field for combustion, so it is a *very* bad choice for your project.

velocity all create \\$t 87287

neighbor 1.5 bin
neigh_modify once yes

not updating the neighbor lists for a liquid system is also a *very* bad idea.

fix 1 all box/relax iso 0.0 vmax 0.001

fix box/relax has *no* effect during dynamics runs.

fix 2 all qeq/reax 1 0.0 10.0 1e-6 param.qeq
fix 3 all nvt temp \\$t \\$t 0.03

a choice of 0.03fs for tdamp is a *bad* choice with a time step of 0.01fs.

timestep \\${dt}

a time step of 0.01fs is rather wasteful for the type of your system
at ambient temperatures. with a proper starting geometry and a
suitable force field, you should be able to run a stable, energy
conserving MD with 0.1fs and possibly even larger (0.2 - 0.25fs).

axel.

Dear Dr. Axel Kohlmeyer,

Thank you for your response and suggestions.

I am using LAMMPS version released on 11 Aug 2017, running on Linux platform.
My system is in solid state, a molecule containing C,H, O atoms placed on a graphene sheet.

Could you please suggest some other force-field for this type of system, i.e., containing C,H, and O atoms?

I could not find any other potential in LAMMPS potential directory for C,H,O.

Kind Regards

Saeed

Dear Dr. Axel Kohlmeyer,

Thank you for your response and suggestions.

I am using LAMMPS version released on 11 Aug 2017, running on Linux
platform.
My system is in solid state, a molecule containing C,H, O atoms placed on
a graphene sheet.

​so why are you then mentioning, that you are working on liquid-soli​d
interface simulations?
you can only get as good advice and the information you provide is.

Could you please suggest some other force-field for this type of system,
i.e., containing C,H, and O atoms?

​this is a subject for a thorough study of the published literature. please
note, that classical force fields cannot be treated like ab initio
calculations, i.e. you need to look for something more specific than the
names of the elements. classical force fields are parameterized to a more
narrow range of compounds and thermodynamics states. that includes also the
reax force field, which has many parameterizations for different kinds of
compounds and conditions. in fact, there are two generations of reax, one
for combustion and one for condensed matter.

I could not find any other potential in LAMMPS potential directory for

C,H,O.

​the LAMMPS potentials folder is not meant to provide a complete repository
of force fields and parameter sets. there are *far* too many out there.​
for reaxff specifically, there is the parameter set database of the van
duin group.

axel.