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 6-31g
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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