Hi users,
I am trying to use the pair_style buck/coul/cut and want to use a different global cut off for Buckingham and Coulomb for SiO2.How can I determine these cutoff values for a box with size Nx=8, Ny=10 and Nz=8.
Also, from the lammps info page(http://lammps.sandia.gov/doc/pair_buck.html) , A, rho, C, cutoff and cutoff2 need to be defined for each pair of atom types while using the pair_coeff command. I have 2 atom types Si and O. So, should I be having 2 pair_coeffs defined?
Attached is my lammps input script with an assumption that the cutoff and cutoff 2 values are the same in the pair_style and pair_coeffs commands. The compiler throws an error - All pair coeffs are not set.
Could some body please clarify this for me.
Regards,
Saketh.
SilicaError.txt (1.36 KB)
Hi users,
I am trying to use the pair_style buck/coul/cut and want to use a different
global cut off for Buckingham and Coulomb for SiO2.How can I determine these
cutoff values for a box with size Nx=8, Ny=10 and Nz=8.
the cutoff is determined by the parametrization, not the box size.
Also, from the lammps info page(LAMMPS Molecular Dynamics Simulator)
, A, rho, C, cutoff and cutoff2 need to be defined for each pair of atom
types while using the pair_coeff command. I have 2 atom types Si and O. So,
should I be having 2 pair_coeffs defined?
no, you need three. one for Si-Si, one for Si-O and
one of O-O. sometimes the parameters for the mixed
set (Si-O) can be derived from mixing rules. not sure
about this case though. the paper where you take/took
the potential parameters from should be explaining this.
Attached is my lammps input script with an assumption that the cutoff and
cutoff 2 values are the same in the pair_style and pair_coeffs commands. The
compiler throws an error - All pair coeffs are not set.
you probably are missing the mixed term.
axel.
Hi
this is wrong:
pair_style buck/coul/cut 10.0
pair_style buck/coul/cut 10.0 8.0
pair_coeff 1 1 1282.9 0.321 10.61 10.0
pair_coeff 1 1 1282.9 0.321 10.61 10.0 8.0
should be like this:
pair_style buck/coul/cut 10.0
pair_coeff 1 1 1282.9 0.321 10.61
pair_coeff 1 2 xxx.xx xxx.xx xxx.xx
pair_coeff 2 2 xxx.xx xxx.xx xxx.xx
It does not really make sense to use a shorter cutoff for coulomb then for the buckingham part (the latter is the long range interaction).
If you want longer rang coulomb interactions do something like:
pair_style buck/coul/cut 10.0 15.0
pair_coeff 1 1 1282.9 0.321 10.61
pair_coeff 1 2 xxx.xx xxx.xx xxx.xx
pair_coeff 2 2 xxx.xx xxx.xx xxx.xx
But using a long range coulomb solver (i.e. pppm) will most likely be faster and more accurate than this.
Cheers
Christian
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