The Rotate animation makes a group of particles rotate around its geometric center. The rotation is done around a vector that goes through the centroid of the group of particles and is collinear with the Z-axis.
See also animations: Rock, Hold atoms, Move atoms
First, select a group of particles that you want to be rotated around their centroid.
Then, double-click on the Rotate animation effect in the Animation panel of the Animator or click Animation menu > Motion > Rotate.
The selected group of particles will be rotated between the two keyframes. Move the keyframes of the animation as needed.
Note: You can always move the keyframes of the animation.
You can change how the parameters are interpolated between the frames by modifying the Easing curve.