# velocity

Dear All,
I am using LAMMPS (14 Feb 2014) and have some questions ：
1.The mom and rot options are used by create style,and they aren’t suitable for normal particles,just for finite-size
particles,right?

2.Manual tells us ‘Since kinetic energy is a function of particle velocity, there is often a need to distinguish
between a particle’s advection velocity (due to some aggregate motiion of particles) and its thermal velocity.
The sum of the two is the particle’s total velocity, but the latter is often what is wanted to compute a
temperature’.
Is so-called ‘particle’s advection velocity’ the one we can see in our real life ,for example ,the velocity of cars?

3.Compute ke/atom command will calculate kinetic energy simply via 1/2 m v^2. Is v the total velocity(sum of particle’s advection velocity and its thermal velocity) of each atom?

4.At 10K, a block(a group of particles) is immobile initially. If I want to give it a velocity to move it in this velocity,will the commands be:
velocity block create 10.0 5812384
velocity block set 14 0.0 0.0 sum no
or

velocity block create 10.0 5812384
velocity block set 14 0.0 0.0 sum yes ?

sum no or sum yes,which one is right?

Dear All,
I am using LAMMPS (14 Feb 2014) and have some questions ：
1.The mom and rot options are used by create style,and they aren't suitable
for normal particles,just for finite-size
particles,right?

wrong!

2.Manual tells us 'Since kinetic energy is a function of particle velocity,
there is often a need to distinguish
between a particle's advection velocity (due to some aggregate motiion of
particles) and its thermal velocity.
The sum of the two is the particle's total velocity, but the latter is often
what is wanted to compute a
temperature'.
Is so-called 'particle's advection velocity' the one we can see in our real
life ,for example ,the velocity of cars?

that is an extreme example and i am not 100% certain what you mean,
but in my understanding: yes.

3.Compute ke/atom command will calculate kinetic energy simply via 1/2 m
v^2. Is v the total velocity(sum of particle's advection velocity and its
thermal velocity) of each atom?

for an individual (point particle), there is no distinction between
the two. there is only *the* velocity. and ke/atom only considers
point particles (be they part of an aggregate or not)

4.At 10K, a block(a group of particles) is immobile initially. If I want to
give it a velocity to move it in this velocity,will the commands be:
velocity block create 10.0 5812384
velocity block set 14 0.0 0.0 sum no
or
velocity block create 10.0 5812384
velocity block set 14 0.0 0.0 sum yes ?
sum no or sum yes,which one is right?

both. but they do slightly different things. you have to choose. the
manual explains the difference.

axel.

For the question 4,I just want to drive the block to move at a velocity of ‘14’ at 10K ,like the real life.I have read the manaul before.‘sum yes’ will add the velocity of ‘14’ to the exsting one and ‘sum no’ will replace it.
I think ‘velocity block create 10.0 5812384’ will create the thermal velocities,and ‘velocity block set 14 0.0 0.0 sum yes’ will add particle’s advection velocities.This method seems OK for what I want above,i.e.driving the block to move at a velocity of ‘14’ at 10K.
However,the other one( velocity block set 14 0.0 0.0 sum no)will give all the particles the same velocity of ‘14’.This seems also reasonable.But I am not sure whether the velocity of ‘14’ contains thermal velocity.If contain,the block will move at a velocity less than ‘14’.I am still confused which one will be right?

For the question 4,I just want to drive the block to move at a velocity of
'14' at 10K ,like the real life.I have read the manaul before.'sum yes' will
add the velocity of '14' to the exsting one and 'sum no' will replace it.
I think 'velocity block create 10.0 5812384' will create the thermal
velocities,and 'velocity block set 14 0.0 0.0 sum yes' will add
particle's advection velocities.This method seems OK for what I want
above,i.e.driving the block to move at a velocity of '14' at 10K.
However,the other one( velocity block set 14 0.0 0.0 sum no)will give
all the particles the same velocity of '14'.This seems also reasonable.But I
am not sure whether the velocity of '14' contains thermal velocity.If
contain,the block will move at a velocity less than '14'.I am still confused
which one will be right?

you *will* not get a "yes" or "no" type of answer from me on any
question of this kind, because you are asking a question that you
should not ask. whether something is the correct way for your specific
purpose or not, is not decided by a committee or pontificated by a
single person (like me), but if you are unsure, *you* have to make a
test and *verify* which of the available options provides you with the
result you want to achieve. this is an essential part of the
scientific process and i would be doing you a disservice by making
that decision for you, however much you would desire it. you later
have to justify your choices and then you cannot say "i didn't do
that, some person on the internet told me".