From NWChem
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9:23:50 PM PST  Tue, Jan 30th 2018 

Hi NWChem users and developers,
I have a condensed matter physics background and I am trying to learn the HF method using the NWChem code. The thing I found is that I can not successfully get a correct UHF solution of the H2 molecule. I suppose that by setting "UHF; singlet" the NWChem code will automatically find the solution that is slightly lower than the "RHF" calculation when the HH distance is larger than 3.0 a.u.. But it turns out that the two calculations give the same energy. I have turned the symmetry off.
Your help is welcomed!
Best,
YunPeng Wang




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Posts 177


6:45:12 AM PST  Wed, Jan 31st 2018 

You need to break the spin symmetry in order to obtain the lower energy solution. By default, NWChem will start with a wave function that is symmetric with respect to spin. Try this input.
echo
start H2
geometry "tot"
H 0.0 0.0 1.6
H 0.0 0.0 1.6
end
geometry "ha"
H 0.0 0.0 1.6
end
geometry "hb"
H 0.0 0.0 1.6
end
basis
H library 631g
end
set geometry "ha"
dft
xc hfexch 1.0
odft
mult 2
vectors output ha.movecs
sym off
adapt off
end
task dft
set geometry "hb"
dft
xc hfexch 1.0
odft
mult 2
vectors output hb.movecs
sym off
adapt off
end
task dft
set geometry "tot"
dft
xc hfexch 1.0
mult 1
vectors input fragment ha.movecs hb.movecs
sym off
adapt off
end
task dft
We first solve for the individual atoms, the first one with the electron in the alpha spin channel and the second one with the electron in the beta spin channel (the negative on the mult keyword specifies that you want the excess electrons in the beta channel rather than the alpha channel). We then combine those wave functions as the initial guess for the molecule.




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5:51:33 AM PST  Mon, Feb 19th 2018 

Dear Dr. Sean
Surely enough, your input does get lower total energy than that obtained without the breaking of the symmetry
Very Best Regards!

Edited On 6:04:08 AM PST  Sun, Feb 25th 2018 by Xiongyan21



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