Implementing coordination number constraint for AIMD

[apolet]

Hi folks,

How does one specify in ICONST the flags to make a coordination number constraint?
I am guessing that it will be some combination of flags W and D, but I do not think I understand how they come together.

I am ideally looking for an example, the same way that the VASP wiki provides helpful examples on the other flags.

The context for this is trying to create a metadynamics-biased simulation of the exchange of coordinating atoms around a central atom.

Thank you!

[toms_bucko1]

Hello Apolet,

perhaps it's best to explain the definition of coordination number (CN) on a concrete example. Assume, for instance, that you want to define a coordination number whose definition involves bonds between the atoms 1 and 6 (numbering refers to the order of appearance in POSCAR), and 1 and 5, and assume that the reference distances are 1.1 and 1.5 {\AA}, respectively. The first way to define CN makes use of the primitive type R and the complex type D - in this case the corresponding ICONST file would be written like this:

R 1 6 0
R 1 5 0
D 1.1 1.5 5

Here, the primitives R merely form the basis for the definition of D and the value of their "STATUS" (0) does not matter, but must be defined. The coefficients appearing in D are the reference bond lengths in {\AA} and the "STATUS" of 5 tells the VASP that the D coordinate is going to be used in meta-dynamics. The exponents appearing in numerator and denominator of the defining formula of D (see https://www.vasp.at/wiki/index.php/ICONST) are, by default, fixed at the values 9 and 14, respectively.

Alternatively (but equivalently), one can define the same CN using the W primitive and S complex coordinates. This can be accomplished as follows:

W 1 6 1.1 9 14 0
W 1 5 1.5 9 14 0
S 1 1 5

Once again, only "STATUS" of the complex coordinate (here S) matters. Notice that, unlike in the previous format, the reference bond lengths are given in definition of primitives W. The advantage of this format is that the coefficients M and N (see the definition of W here https://www.vasp.at/wiki/index.php/ICONST) can be set for each distance separately. Moreover, this format allows for an easy and intuitive definition of (possibly scaled) sums of (and/or differences) between multiple coordination numbers - simply via a suitable choice of coefficients linked with S, which can be positive, negative or zero.

I hope this helped! We updated the relevant wiki page (https://www.vasp.at/wiki/index.php/ICONST) to include this information.

[apolet]

Yes, this is indeed very helpful. Thank you for the explanation!