I've only recently began using quantum espresso to run a simulation on carbon nanotubes. After doing a few scf calculations until total energy had converged I ran a projwfc.x calculation and output a few lowdin charges. My CNT structure is functionalized with just simple hydrogens, however when I look at my lowdin charges from the projwfc.x output I'm getting total lowdin charges for the hydrogen of ~.7 as shown below..
Atom # 109: total charge = 0.7422, s = 0.7422,
Atom # 109: total charge = 0.7422, p = 0.0000, pz= 0.0000, px= 0.0000, py= 0.0000,
If I understand lowdin charges correctly to get the partial charge from it I subtract the expected valence electrons which in the case of hydrogen is 1 and I'd get a partial charge of ~+.26 on the hydrogen bonded to a carbon. Being that Carbon and hydrogen have approximately equal electronegativity I'd expect this value to be closer to 0. Am I misinterpretting that data? What would be the proper way to obtain a partial charge from the projwfc.x output?
There is no universially agreed upon way to partition electron densities or compute partial charges. So for one, I would not put too much stock in absolute values. That being said, relative trends are usually physically sensible for most methods and systems. Here, you find that H atoms have a slight positive charge, which I think is usually assumed to be true if they are bound to sp2 carbon.
Thanks Kristof M. Bal for the answer. Just a quick follow up that I'm hoping you can answer, when I ran projwfc.x on the same structure and got those lowdin charges, I notice that the total charge on the entire system was not 0 as expected.After totalling up the total charge values of lowdin charges and accounting for valence electrons of course, I end up with a total charge of ~ -10. Why would this be the case in what should be an electrically neutral system? Is there a setting I can apply to force a neutral charge?
This seems like a convergence problem w/r/t the density, but I don't use QE myself so I don't know the specifics.
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