Small measurements modifications.

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[1] Double occupancy measurement should be added.
[2] Total energy measurement should be changed to match most intuitive 
definition of the total energy.

Original issue reported on code.google.com by iglovi...@gmail.com on 7 Jan 2014 at 9:47

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[3]Magnetisation squared should be also added.

m^2 = n_up^2 + n_down^2 - 2 n_up n_down

Original comment by iglovi...@gmail.com on 17 Jan 2014 at 8:31

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Double occupancy is the density-density correlation at zero separation. 
In QUEST equal-time measurement output, the correlation is in the section:
"Density-density correlation fn: (up-dn)"

Original comment by cxc639 on 9 Feb 2014 at 12:31

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I agree, double occupancy can be extracted from the "Density-density 
correlation fn: (up-dn)".

But:
[1] Because this quantity is important, I believe that it should be represented 
by a separate variable.
[2] Let's say I have 3 sites in the unit cell. site 0, 1 and 2. Double 
occupancy will be <n_1up n1_down> + <n2_up n2_down> + <n3_up n3_down>, this is 
clear. But how do you want to get errorbars of this quantity?

Original comment by iglovi...@gmail.com on 9 Feb 2014 at 4:12

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To answer [2], an estimate of error (not the best one) can be done by using 
error propagation formula.

Original comment by cxc639 on 13 Feb 2014 at 7:20

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Fixed in revision 9531f929655c  

Original comment by iglovi...@gmail.com on 11 Apr 2014 at 9:44

• Changed state: Fixed