Waht does the "cdft 1 2 charge 1.0" mean?


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Just Got Here
11:26:00 PM PST - Wed, Nov 21st 2012
In the documentation of NWchem 5.1.1 p128, there is a constrained DFT example as:

geometry
symmetry
C ..
O ..
C ..
O ..

basis
..
end

dft
...
cdft 1 2 charge 1.0
end
task dft

What does the colored line mean?