Dear LAMMPS users and developers,
LAMMPS version: 18 April 2015
I am in the process of using pair_style lj/expand for some test simulations. During my tests, I found that the tail corrections for energy and pressure were unreasonably high for WCA like cut-offs. This lead me to read the file pair_lj_expand.cpp . According to the manual (as well as the code), the actual force cut-off is the user provided cut-off (Rc) + the shift factor (Delta). However, this definition of cut-off is not reflected in the tail corrections as coded in init_one().
In particular, I find that line 226 should be: “double shiftcut = - cut[i][j];” instead of: “double shiftcut = shift[i][j] – cut[i][j];” Based on some basic tests, I find that the modified code shows expected behavior.
I have attached the derivation for the tail corrections contribution for the energy term. I believe that the source of the error is that the integration limits were not adjusted during the integration-by-substitution (highlighted). Could you please advise whether there is indeed an error in the pair_lj_expand.cpp or I am missing something? Also, it seems as though pair_lj_expand_omp.cpp inherits init_one() from pair_lj_expand.cpp … so only one line might need to be changed.
Thanks,
Ketan Khare
Rensselaer Polytechnic Institute
Tail correction LJ Expand.pdf (222 KB)
Apologies for resending, sending it as plain text.
Dear LAMMPS users and developers,
LAMMPS version: 18 April 2015
I am in the process of using pair_style lj/expand for some test
simulations. During my tests, I found that the tail corrections for energy
and pressure were unreasonably high for WCA like cut-offs. This lead me to
read the file pair_lj_expand.cpp . According to the manual (as well as the
code), the actual force cut-off is the user provided cut-off (Rc) + the
shift factor (Delta). However, this definition of cut-off is not reflected
in the tail corrections as coded in init_one().
In particular, I find that line 226 should be: double shiftcut = -
cut[i][j]; instead of: double shiftcut = shift[i][j] cut[i][j]; Based
on some basic tests, I find that the modified code shows expected behavior.
I have attached the derivation for the tail corrections contribution for the
energy term. I believe that the source of the error is that the integration
limits were not adjusted during the integration-by-substitution
(highlighted). Could you please advise whether there is indeed an error in
the pair_lj_expand.cpp or I am missing something? Also, it seems as though
pair_lj_expand_omp.cpp inherits init_one() from pair_lj_expand.cpp
so only
one line might need to be changed.
Thanks,
Ketan Khare
Rensselaer Polytechnic Institute
Tail correction LJ Expand.pdf (222 KB)