Frequently Asked Questions

Does the H in COHP stand for "Hamilton" or "Hamiltonian"?

It stands for Hamilton; crystal orbital Hamilton population was the "official" name given. Some people, however, believe that the tool is important enough to deserve that extra "ian". COHP results are independent from your spelling choice.

What is the difference between COOP and COHP?

Both COOP (crystal orbital overlap population) and COHP (crystal orbital Hamilton population) are partitioning methods for analyzing the (k-dependent) wavefunction. While COOP partitions the electron number, COHP partitions the band structure energy. As a consequence, if you calculate the energy integral of a COOP curve, you get a number of electrons (like in the Mulliken scheme); if you integrate a COHP curve, you get an energy value that hints toward the bond strength.

You say "hints toward". Why so careful wording?

We are careful in our wording, on purpose. The integrated COHP does not simply equate to a "bond strength" for a number of reasons (for example, it partitions the one-particle eigenvalues and not the total energy of your system). Please keep this in mind when computing integrated COHPs. If put in the right perspective, they are nonetheless highly instructive (we think)!

I have just found an exciting new material and want to study its chemical bonding properties; do I have to run a self-consistent calculation before plotting COHPs?

Yes, unfortunately :-) The electronic structure must have been calculated before the chemical information can be extracted, traditionally with tight-binding codes such as TB-LMTO-ASA. We have recently transferred the method to plane-wave basis sets, too (see the Theory section), and you can even get a program here which will digest your VASP output.

So LMTO is obsolete?

By all means, no! We still use this code in our own research, for questions to which it is well suited. Nonetheless, we believe that it will be quite beneficial to have "projected COHPs" available, too, because they allow to deal with a much wider scope of chemical applications.