Open-shell Systems¶

(Reference: tests/CI_test/VMC-HNO-cipsi_322_dets_143_csfs_noopt)

Open-shell systems (molecules with unpaired electrons) require special attention to spin states.

Configuration¶

1. Spin Polarization:

   • Specify the number of up (nup) and total (nelec) electrons correctly in the input.
   • \(N_{dn} = N_{elec} - N_{up}\).

2. Multi-Determinant Wavefunctions:

   • Open-shell systems often require multi-determinant wavefunctions (e.g., from CIPSI or CASSCF) to capture static correlation.
   • Ensure the expansion includes the necessary determinants for the target spin state.

Example (HNO)¶

HNO radical has 7 up and 6 down electrons (with pseudo-potentials).

load molecule
    ...
end

%module electrons
    nup         7
    nelec       13
%endmodule