Hello all,

I am calculating A * exp(-rij/rho)) - (C/(rij**6) (buck)+ 4 * eps * ( ( sig/rij )**12 - (sig/rij)**6 ) (lj) Vs coulombic (qiqj/rij) interaction energies by hand and would like to compare them to my simulation in lammps. Pair potentials as given below :

pair_style hybrid buck/coul/long 8.0 lj/cut/coul/long 8.0

pair_coeff 1 1 buck/coul/long 717647.4 0.154 121.0676

pair_coeff 2 2 buck/coul/long 271716.3 0.234 696.8883

pair_coeff 1 2 buck/coul/long 391049.1 0.194 290.3317

pair_coeff 1 3 buck/coul/long 28593.02 0.265 148.000

pair_coeff 2 3 buck/coul/long 271716.3 0.234 696.8883

pair_coeff 2 4 buck/coul/long 7088.0 0.2563 51.975

pair_coeff 3 3 lj/cut/coul/long 0.1553 3.166

pair_coeff 1 4 lj/cut/coul/long 0.00144 3.7475

pair_coeff 4 4 lj/cut/coul/long 0.00 0.0

what would be the units of these energies calculated Vs those as printed out by thermo_style multi ? Particlularly ecoul and buckingham(both as compared to say kcal). And how are the conversions done ? Am sorry but I couldnt follow/undrstand the doc respective page.

http://lammps.sandia.gov/doc/units.html

Sincerely,

Ritwik.

Hello all,

I am calculating A * exp(-rij/rho)) - (C/(rij**6) (buck)+ 4 * eps * ( (

sig/rij )**12 - (sig/rij)**6 ) (lj) Vs coulombic (qiqj/rij) interaction

energies by hand and would like to compare them to my simulation in lammps.

[...]

what would be the units of these energies calculated Vs those as printed out

by thermo_style multi ? Particlularly ecoul and buckingham(both as compared

to say kcal). And how are the conversions done ? Am sorry but I couldnt

you didn't tell what is the units setting that _you_ are using.

everything depends on that. depending on that, input and output

use the same units.

follow/undrstand the doc respective page.

http://lammps.sandia.gov/doc/units.html

_what_ did you not understand? where is the problem?

cheers,

axel.

Hello,

sorry about that. my units are:

units real

the doc page says

For units lj

mass, sigma, epsilon, and the Boltzmann constant = 1 (what is sigma and epsilon here ?)

energy = epsilon, where E* = E / epsilon

And for units real

energy = Kcal/mole. (does that mean the print from thermo_style multi gives ecoul and other energy terms in kcal/mole ?)

Thus are there any conversion factors needed for the formulae below to get the same units as in lammps. (no we are not comparing the values, but I would like to get the same units used in lammps for my interactions calculations)

A * exp(-rij/rho)) - (C/(rij**6) (buck)+ 4 * eps * ( ( sig/rij )**12 - (sig/rij)**6 ) (lj) and coulombic (qiqj/rij)

thank you,

Ritwik

Hello,

sorry about that. my units are:

units realthe doc page says

For units lj

mass, sigma, epsilon, and the Boltzmann constant = 1 (what is sigma and

epsilon here ?)

energy = epsilon, where E* = E / epsilon

but you are _not_ using lj units (which are reduced units), so this

does not apply.

And for units real

energy = Kcal/mole. (does that mean the print from thermo_style multi gives

ecoul and other energy terms in kcal/mole ?)

Thus are there any conversion factors needed for the formulae below to get

if you set units real, then you have to give input energies in

kcal/mol, distances

in angstrom and so on, and you get output energies in kcal/mol, angstrom and

so on.

the same units as in lammps. (no we are not comparing the values, but I

would like to get the same units used in lammps for my interactions

calculations)

A * exp(-rij/rho)) - (C/(rij**6) (buck)+ 4 * eps * ( ( sig/rij )**12 -

(sig/rij)**6 ) (lj) and coulombic (qiqj/rij)

well, you have to look or figure out in which units A, C and so

on are and then you should get the same in your test calculations.

coulomb is a special case, since the typical formula usually skip

the conversion factor for elementary charges^2/angstrom to energy.

but that is less of a problem for lammps but rather for your

manual calcuation.

cheers,

axel.