#! Benzene vertical singlet-triplet energy difference computation, using the
#! PubChem database to obtain the initial geometry, which is optimized at the HF/STO-3G
#! level, before computing single point energies at the RHF, UHF and ROHF levels of theory.

memory 250 mb

molecule benz{
    pubchem:benzene
}

set {
   basis     sto-3g
   reference rhf
   max_energy_g_convergence 8
}

optimize('scf')

set basis cc-pvdz
e_sing_rhf = energy('scf')

benz.set_multiplicity(3)
#
# We could have specified a triplet C6H6 molecule with the following
#
#molecule benz {
#    0 3
#    pubchem:benzene
#}
set reference rohf
e_trip_rohf = energy('scf')
set reference uhf
e_trip_uhf  = energy('scf')

vertical_uhf  = psi_hartree2ev * (e_trip_uhf  - e_sing_rhf)
vertical_rohf = psi_hartree2ev * (e_trip_rohf - e_sing_rhf)
print_out("\nSinglet-Triplet gap (vertical, UHF)   = %8.2f eV\n" % vertical_uhf)
print_out("\nSinglet-Triplet gap (vertical, ROHF)  = %8.2f eV\n" % vertical_rohf)

