Computing <S^2> on an FCI calculation

Hello! According to the manual, you can ask for a <S^2> computation using the option CALC_S_SQUARED, together with ICORE=1.

However, when set these options the output doesn’t show it. How can I get this quantity?

I’m using Psi4 v1.5.


Try turning the print level up.

Thank you for the fast response!

I went to print level 8 using the print variable. A lot of information is shown, including the full eigenvectors (millions of lines!). But no S^2.

Line 3144 of reads:

if (print_lvl_) outfile->Printf("Computed <S^2> vector %d = %20.15f\n\n", vec_num, S2);

But it seems that the PRINT variable doesn’t set print_lvl_?

I naively thought that turning the print level up would increase print_lvl_, but no, a deeper look at the code tells me that variable is locked at 0 and never ever changed. Unfortunately, there isn’t a way to look at the value of S^2 in Psi4 1.5. I’ll file an issue for this to be fixed in a future release.

Thanks for the bug report.