Units for K in spring/self

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1

Dear LAMMPS Community,

I’d like to confirm something basic; are the units of K in fix spring/self eV/angstrom^2 or eV/(units lattice)^2.

Thank you,

Tara

2

Dear LAMMPS Community,

I'd like to confirm something basic; are the units of K in fix spring/self
eV/angstrom^2 or eV/(units lattice)^2.

K = spring constant (force/distance units)

​force and distance units depend on your units setting.

axel
​

3

Thank you Professor Kohlmeyer. I apologize for not being more specific. I am using metal units. Other LAMMPS commands that I have used, such as velocity, explicitly state that the default is in lattice units. fix spring/self doesn’t specify this so I assumed that spring/self, for metal units, is in eV/Angstrom^2. But I am not sure if I am misunderstanding the documentation.

4

Thank you Professor Kohlmeyer. I apologize for not being more specific. I
am using metal units. Other LAMMPS commands that I have used, such as
velocity, explicitly state that the default is in lattice units. fix
spring/self doesn't specify this so I assumed that spring/self, for metal
units, is in eV/Angstrom^2. But I am not sure if I am misunderstanding the
documentation.

the easiest and best way to address your concerns is to make a test. never
trust somebody's say-so unless you can confirm it! remember, that it is you
that is being held responsible for the correctness of your work. shifting
the blame on some random person on the internet that you don't know in
person is not going to be accepted.

in this case, a test is extremely trivial to do. set up a simple system and
monitor the restoring force(s) of fix spring/self. now run the same input,
but change the lattice spacing after you have set up your system, but
before you define fix spring/self. whether this results in the same forces
or not will give you the answer you want. no more need to go on the say-so
of somebody else.

​all of the commands that allow (and often default to) using lattice
spacing for length scales​ *do* document this (and have a units option).
only features that are documented exist. if you come across a feature that
is not documented, it would be considered a bug.

axel.