correct way to implement gravity

Submitted by kiwikid on Mon, 07/10/2017 - 23:25

Hi there,

I am getting a "WARNING: One or more atoms are time integrated more than once" which I believe I have tracked down to two lines:

fix fu all move variable NULL NULL NULL NULL v_Vy NULL units box
fix gravity all gravity 9.81 0 -1 0

i.e., I am doing a gravity and also a forcing via a variable Vy. My results are looking a bit odd, so I am wondering if I should instead incorporate the gravity into the fix move? i.e. making the value "v_Vy - 9.81*time" instead of "v_Vy"?

Or is it fine to have this warning?

Forums:

mschramm | Tue, 07/11/2017 - 00:13

fix fu will not allow your particles to have a velocity in the y-direction other than v_Vy and by not setting an explicit setting for the x-displacement, it is time integrated.
This is probably why you are getting this warning.

Are you just trying to give your particles an initial velocity?

Using your option to have your Vy variable be Vy-9.81*time should give you what you are looking for.