# coul/long and coul/cut

Dear lammps-users
From manual,we know:
Styles lj/cut/coul/long compute the same Coulombic interactions as style lj/cut/coul/cut except that an additional damping factor is applied to the Coulombic term so it can be used in conjunction with the kspace_style command and its ewald or pppm option. The Coulombic cutoff specified for this style means that pairwise interactions within this distance are computed directly; interactions outside that distance are computed in reciprocal space.
question 1
if we use style lj/cut/coul/long , do we must use kspace_style? why this type should use kspace_style?
question 2
if we use style lj/cut/coul/cut*,we dont need to use* kspace_style command? How Coulomb force calculated？
why is that?
question 3
Is lj/cut/coul/long or lj/cut/coul/cut impact on the result significant?
Thanks!
Jack zhuang

Dear lammps-users
From manual,we know:
Styles lj/cut/coul/long compute the same Coulombic interactions as style
lj/cut/coul/cut except that an additional damping factor is applied to the
Coulombic term so it can be used in conjunction with the kspace_style
command and its ewald or pppm option. The Coulombic cutoff specified for
this style means that pairwise interactions within this distance are
computed directly; interactions outside that distance are computed in
reciprocal space.
question 1
if we use style lj/cut/coul/long , do we must use kspace_style? why this
type should use kspace_style?

you gave the short version of an answer in the previous paragraph. for
details, please consult a proper text book about MD and force fields.

question 2
if we use style lj/cut/coul/cut,we dont need to use kspace_style command?
How Coulomb force calculated？
why is that?

same answer as above. again, please consult a proper text book. this
is basic stuff that you should be trained in (or train yourself in)
*before* even looking at a code like LAMMPS or how to set up
calculations. without this knowledge you are bound to make many
mistakes, most of which are not likely to be flagged by the MD code
(since they are mathematically still valid, but physically
meaningless).

question 3
Is lj/cut/coul/long or lj/cut/coul/cut impact on the result significant?

depends. generally, people use the long version only when the impact
on accuracy needs it (which is frequent).

axel

-question 1

-if we use style lj/cut/coul/long , do we must use kspace_style? why this type should use kspace_style?

Answer 1: There are several methods to calculate the Coulombic interactions in the reciprocal space (kspace) i.e. the long range Coulombic interactions.

-question 2

-if we use style lj/cut/coul/cut,we dont need to use kspace_style command? How Coulomb force calculated？

-why is that?

Answer 2: The Coulombic interactions are truncated at cut-off. The long range contribution is neglected/ignored/truncated and so no kspace methods are necessary.

-question 3

-Is lj/cut/coul/long or lj/cut/coul/cut impact on the result significant?

Answer 3: For most simulations containing atoms with partial charges, for most normal cut-offs (less than three sigma), the truncating Coulombic interactions rather than accounting for them would lead to discrepancies of a similar magnitude as the Coulombic energy itself.

-question 1

-if we use style lj/cut/coul/long , do we must use kspace_style? why this type should use kspace_style?

Answer 1: There are several methods to calculate the Coulombic interactions in the reciprocal space (kspace) i.e. the long range Coulombic interactions.

-question 2

-if we use style lj/cut/coul/cut,we dont need to use kspace_style command? How Coulomb force calculated？

-why is that?

Answer 2: The Coulombic interactions are truncated at cut-off. The long range contribution is neglected/ignored/truncated and so no kspace methods are necessary.

-question 3

-Is lj/cut/coul/long or lj/cut/coul/cut impact on the result significant?

Answer 3: For most simulations containing atoms with partial charges, for most normal cut-offs (less than three sigma), the truncating Coulombic interactions rather than accounting for them would lead to discrepancies of a similar magnitude as the Coulombic energy itself.

-question 1

-if we use style lj/cut/coul/long , do we must use kspace_style? why this type should use kspace_style?

Answer 1: There are several methods to calculate the Coulombic interactions in the reciprocal space (kspace) i.e. the long range Coulombic interactions.

-question 2

-if we use style lj/cut/coul/cut,we dont need to use kspace_style command? How Coulomb force calculated？

-why is that?

Answer 2: The Coulombic interactions are truncated at cut-off. The long range contribution is neglected/ignored/truncated and so no kspace methods are necessary.

-question 3

-Is lj/cut/coul/long or lj/cut/coul/cut impact on the result significant?

Answer 3: For most simulations containing atoms with partial charges, for most normal cut-offs (less than three sigma), the truncating Coulombic interactions rather than accounting for them would lead to discrepancies of a similar magnitude as the Coulombic energy itself.