Molcas Forum

Okko

Member

Registered: 2023-04-27

Posts: 11

Analysis of spin-orbit coupling effects: RASSI vs. SO-RASSI

Dear all,

I am interested in the investigation of the spin-orbit coupling effect and therefore I am comparing the lowest three excited states of the oxygen and water molecule. The number of JOBMIX files corresponds to 6 for oxygen and water in order to consider several electronic configurations (singlet, triplet and quintet excited states for both symmetries). The RASSI and SO-RASSI state results for O2 are:

::    RASSI State    1     Total energy:    -149.47791996746432
::    RASSI State    2     Total energy:    -149.26684257981680
::    RASSI State    3     Total energy:    -149.26468394267550

       Total energies including SO-coupling:
::    SO-RASSI State    1     Total energy:    -150.05808664956120
::    SO-RASSI State    2     Total energy:    -150.05807602651814
::    SO-RASSI State    3     Total energy:    -150.05807602541822

and for H2O:

::    RASSI State    1     Total energy:     -75.49656390919183
::    RASSI State    2     Total energy:     -74.86295455475562
::    RASSI State    3     Total energy:     -74.33449216107643

       Total energies including SO-coupling:
::    SO-RASSI State    1     Total energy:     -76.37844173264253
::    SO-RASSI State    2     Total energy:     -76.03235738312220
::    SO-RASSI State    3     Total energy:     -76.03235738312220

Comparing the first energies, i.e., CASSI vs SOCASSI results in: ∆E =-0.580166682 a.u. (O2) and -0.881877823 a.u. (H2O)

Since these molecules are considered to include light elements the SOC effect should be negligible. However, these values seem too high in the unit of eV. Hence, I am not sure whether my approach is misleading or what would be the right approach for studying the SOC effect?

I am gratefuly for any hint!

Thanks in advance!