[Pw_forum] Cohesive Energy of N2 molecule

Masoud Nahali masoudnahali at gmail.com
Tue May 3 19:56:01 CEST 2011

> Izaak Williamson wrote on Tue, 3 May 2011 09:54:14 -0700
> I am trying to calculate the cohesive energy of the N2 molecule using the
> attached input file (relax.in) and am getting a value of -16.57 eV. I use
> with GGA and the pseudo-potential N.pbe-rrkjus.UPF. Other work [Fuchs et.
> al.,
> Phys. Rev. B 65, (2002)245212] has performed similar calculations and
> obtained
> values ~10.5 eV. They even list an experimental value of 9.76 eV. Why is my
> value so much higher? Is there anything in my input file that could be
> giving
> inaccurate results? Is it my pseudo-potential that is causing this problem?
> Thanks for any help.
> --
> Izaak Williamson
> Research Assistant
> Physics Department
> Boise State University
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Dear Izaak

Did you considered symmetry for energy calculation of atomic nitrogen or not
? As you know the electronic configuration of atomic nitrogen is 1s[2] 2s[2]
2px[1] 2py[1] 2pz[1] and seems symmetric. In some quantum mechanical codes
when one calculates the energy of this system without considering symmetry
may get wrong answer since the system may go to a non-symmetric status like
1s[2] 2s[2] 2px[2] 2py[1]. Please calculate the energy with and without
symmetry and compare them.
Another important issue is that did you considered polarization of spin
(with defining a proper starting magnetization value for nitrogen) in your
calculations ? It can cause a significant change in the energy since your
system includes unpaired electrons.
Let us know the changes after considering the above issues. I hope it helps.

Best Wishes


Masoud Nahali, Sharif University of Technology

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