Dear all

In the NEB calculation, we always find a distance between images, then how to define this distance? or how is this distance calculated?

thanks

## How is the distance between images defined ?

**Moderator:** moderators

acos(v_next.v_prev)

where v_next is a unit vector to the next image, and v_prev is a unit vector to the previous image.

The mass does not play any roll. There is no mass weighting, or zeroing of the center of mass. Very often we use frozen atoms when modeling slabs, so that it doesn't make sense to freeze the center of mass. If no atoms are frozen, the vasp optimizers will zero out any motion of the center of mass. This constraint, however, may not work with the NEB, and we do not make any checks or assumptions about the center of mass.

acos(v_next.v_prev)

where v_next is a unit vector to the next image, and v_prev is a unit vector to the previous image.

[/quote]

tanks again.

How can we determine the unit vector? In my guess, we can get a vector between corresponding atoms in two connected images, adding them to get the total vector and then choose a unit vector along the direction of the total vector, is that right? I am not sure I have made it clear here.

another question, what is the physical meaning of the angle?

We don't use this angle in the NEB calculation. It is just printed to give a measure of how straight the band is. An angle of 180 means that all atoms are moving in a straight line, and an angle of 0 means that all moving atoms have turned a corner.

There is nothing odd about these quantities, they are just normal vector quantities in a high dimensional space.