How is the distance between images defined ?

Vasp transition state theory tools

Moderator: moderators

Post Reply
hakuna
Posts: 54
Joined: Sat Oct 29, 2005 12:52 am

How is the distance between images defined ?

Post by hakuna »

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
graeme
Site Admin
Posts: 1998
Joined: Tue Apr 26, 2005 4:25 am
Contact:

Post by graeme »

Iit is the root-mean-squared distances between all atoms.
hakuna
Posts: 54
Joined: Sat Oct 29, 2005 12:52 am

Post by hakuna »

[quote="graeme"]Iit is the root-mean-squared distances between all atoms.[/quote]


Thank you, Graeme

then how about the angle between the images? is it the angle between the mass center connections?
graeme
Site Admin
Posts: 1998
Joined: Tue Apr 26, 2005 4:25 am
Contact:

Post by graeme »

The angle between images is

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.
hakuna
Posts: 54
Joined: Sat Oct 29, 2005 12:52 am

Post by hakuna »

[quote="graeme"]The angle between images is

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?
graeme
Site Admin
Posts: 1998
Joined: Tue Apr 26, 2005 4:25 am
Contact:

Post by graeme »

No, these are 3xN dimensional vectors, where N is the number of atoms. There is no summation over atoms.

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.
Post Reply