# Ellipsoids

Dear Lammps Users,

I have some questions regarding the ellipsoids package. Is it possible to compute the ellipsoid principal axis orientation at each time step. I know that there is a compute command that computes quaternions. Is it possible to get the principal axis orientation at each time step. Also i need to connect the ellipsoids with bonds, I know this can be done with hybrid atom style. Is it possible to restrict the rotation of the ellipsoids around their center of mass.

Regards,
Elena

Dear Lammps Users,

I have some questions regarding the ellipsoids package.

​there is no ellipsoids package in LAMMPS, it is called asphere​.

Is it possible to compute the ellipsoid principal axis orientation at each
time step. I know that there is a compute command that computes
quaternions. Is it possible to get the principal axis orientation at each
time step.

the documentation states what is available. the quaternions represent the
property you want. if you want to represent it differently, you would have
to convert them.​

Also i need to connect the ellipsoids with bonds, I know this can be done
with hybrid atom style. Is it possible to restrict the rotation of the
ellipsoids around their center of mass.

no. ​bonds in LAMMPS assume to be connected to point particles, so they'll
also be connecting to the center of objects, including ellipsoids, which
lets them rotate freely.

​if i remember correctly, the question of how to connect ellipsoids with
bonds to off-center ​locations has come up before, and my recollection of
one viable approach would be to build the interacting ellipsoids as rigid
particles from multiple particles of different size and mass, so that the
sites for connecting the bonds can be off-center, yet the non-bonded
interaction would still be aspherical.

​axel.​

Hi Axel,

Thanks for your suggestion . From your reply, I understand that the best way is to build the ellipsoid as rigid body particle which is made of N sub particle and I can choose the location of one of sub particles where I can attach the bond. Am I correct.

Regards,
Elene

Hi Axel,

way is to build the ellipsoid as rigid body particle which is made of N sub
particle and I can choose the location of one of sub particles where I can
attach the bond. Am I correct.

​i cannot say whether this is the best way, but this is what i am
recounting from previous discussions on the mailing list archive.
whether it can work, depends largely on the specifics of your model, and
particularly what rules you have for exclusions.
​you are definitely entering the more dusty and less traveled regions of
the LAMMPS code, so watch out for the "here be dragons" signs...

axel.

Thanks Axel ,

If you want bonds attached to the center

of an ellipsoid, then you can just assign

bonds like you would for any other atom

or spherical particle. If you want a bond

attached at some off-center point, e.g.

the end of the ellipsoid, then you need

to have another particle with its center there.

E.g. make a rigid body with a 2nd particle

(nearly massless would be OK) at the position

you want the bond.

Steve