potential energy of interaction using DFT !!

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Hi,
I am trying to get the interaction-potential (hydrogen bond) between 2 monomers using DFT with BSSE correction.
For the above purpose I have designed the following code:
title 'bond_energy'
start dimer
geometry units angstrom
  #symmetry c1
  N 2.739 24.753 58.631
  H 2.603 25.170 59.371
  O 2.533 26.305 61.093
  C 2.375 27.069 62.059
end
basis "ao basis"
  N library 6-31g
  H library 6-31g
  O library 6-31g
  C library 6-31g
  bqN library N 6-31g
  bqH library H 6-31g
  bqO library O 6-31g
  bqC library C 6-31g
end
dft
  xc m06-2x
  direct
  #convergence energy 1e-8
  iterations 6000
end
bsse
  mon first 1 2
  mon second 3 4
end
task dft energy


So, from this code I'm getting an energy value of -1.42 kcal/mol.
Now, subtracting the sum of isolated energies of monomers from the total-bsse-energy, I got this energy value of 0.00226 Hartree.
Will anyone please tell me is it the right way or i'm heading the wrong way... ??
please reply...
Thank you.
Edited On 12:49:59 AM PST - Mon, Mar 2nd 2015 by Neo

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0.00226 Hartree = 1.42 kcal/mol, so sounds consistent to me.

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Posts 69
thanks for your opinion.
But I was expecting a little more code-oriented comment.
I mean whether it is the energy the code gives or i'm missing some calculative part.

thanks again.

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Well, if you would have tried the BSSE calculation manually, you should have got the same answer as you get with the BSSE keyword, which would have validated your example input. So yes, the input file is correct. The BSSE keyword just simplifies the procedure. It even explicitly prints out the BSSE error/correction.

The convergence of this system, specifically the NH fragment, seems to be a bit problematic for NWChem, however, so depending on your starting guess you will get slightly different answers for the monomer energy with ghost atoms (I saw differences up to 6 microH). In principle, it should work, though.
Edited On 5:13:39 PM PST - Wed, Mar 4th 2015 by Mpjohans

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Posts 69
ok. thanks for the help.
Keeping your point in mind, in some cases of calculation with other monomers (looks exactly like this, only coordinate differs), the energy is getting highly deviated.
So, when I was going through the convergence steps I got some errors showing
Singularity in Pulay matrix. Error and Fock matrices removed.
Although the convergence did not failed in total.
So, any idea what this error/warning message means and should I consider those energies as valid ones ?
Edited On 7:34:51 AM PST - Thu, Mar 5th 2015 by Neo


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