Problem oxygen potential energy scan using CCSD(T) and EA state PES using EA-EOM-CCSD

Questions
#1
  1. I’m calculating PES of O2 using CCSD(T).
    I’m using the following input

$molecule
0 3
O
O 1 1.213185
$end

$rem
jobtype = pes_scan
frozen_scan = true
method = ccsd(t)
basis = aug-cc-pvtz
n_frozen_core = 0
scf_convergence = 8
max_scf_cycles = 500
cc_t_conv = 6

!ccman2 = true
$end

$scan
stre 1 2 0.80 8.0 0.10
$end

Problem: From the PES scan data when I’m trying to calculate Dissociation energy I’m getting DE as ~20 eV. Experimental data reported on NIST is ~5.11 eV.
What am I doing wrong?
Note: Same calculation I have done using ORCA 5.0. There I’m getting DE as ~5.4 eV which is close to experiment.

  1. I’m calculating EA states of O2 using EA-EOM-CCSD
    I’m using the following input

$molecule
0 3
O
O 1 1.213185
$end

$rem
jobtype = pes_scan
frozen_scan = true
method = eom-ccsd
basis = aug-cc-pvtz
n_frozen_core = 0
scf_convergence = 8
max_scf_cycles = 500
cc_t_conv = 6

ea_states = [4,2,3,3,2,3,3,3]
!ccman2 = true
$end

$scan
stre 1 2 0.85 3.1 0.11
$end

Problem: I’m unable to go beyond 3.1 Angstrom , I want the PES till 8.0 Angstrom.
What am I doing wrong?

#2

UHF-CCSD(T) cannot describe the bond dissociations of triplet O2 or doublet [O2]-. These two problems are of significant multiconfigurational characters. It would be better to use multiconfigurational/multi-reference methods like CASSCF, CASPT2, NEVPT2, etc.