CIPSI 2-state calculations
You can import the single-determinant wave function from the provided
as:
qp_import_trexio.py COH2.trexio -o COH2
qp set_file COH2
Specify that you want to run a two-state calculation:
qp set determinants n_states 2
Tell QP to stop when the number of determinants is larger than 2000
qp set determinants n_det_max 2000
and run the CIPSI in the Full-CI space:
qp run fci | tee COH2.fci.out
The extrapolated excitation energy is around 8 eV and we expect 4 ev, so we did not catch the correct state. This is because the orbitals in the TREXIO file are symmetry adapted, so it is impossible to make a determinant from another symmetry enter in the determinant space.
To obtain a solution from another symmetry, we need to put at least one determinant of each symmetry.
Tip
The simplest practical solution is to first perform a CIS, and then continue with a CIPSI in the FCI space.
By default, at every iteration QP tries to double the size of the wave function. In QMC, we will use a small number of determinants, so we should tell QP to add less determinants at each iteration to have a finer selection.
qp set determinants selection_factor 0.5
qp run cis | tee COH2.cis.out
qp set determinants read_wf true
qp run fci | tee COH2.fci.out
read_wf = true specifies that the wave function stored in the EZFIO database should be used as a starting point for the the CI calculation.
Now, we have obtained a more reasonable excitation energy, around 4 eV. We are now ready to export the data for CHAMP.