[lammps-users] units and verifications

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 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

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.