# The oxygen dissociation curve calculated by wB97M-V is not smooth

Dear all,
I am trying to verify the spin selection for the hydrogen combustion system with the wB97M-V method. For example, I choose three ways to calculate the energy with the wB97M-V method and cc-pVTZ basis set: spin=1, spin=3, and guess=mix, and the lowest energy was chosen to draw the dissociation curve. The dissociation curve is not smooth. Here is one input of my input files:

spin =1 or 3:
\$molecules
0 1(3)
O 11.9573 7.97905 12.3234
O 12.4226 6.78293 12.3816
\$end
\$rem
JOBTYPE FORCE
MEM_TOTAL 24000
METHOD wB97M-V
BASIS cc-pVTZ
UNRESTRICTED true
SYMMETRY false
SYM_IGNORE true
\$end

guess=mix:
\$molecules
0 1
O 11.9573 7.97905 12.3234
O 12.4226 6.78293 12.3816
\$end
\$rem
JOBTYPE FORCE
MEM_TOTAL 24000
METHOD wB97M-V
BASIS cc-pVTZ
UNRESTRICTED true
SYMMETRY false
SYM_IGNORE true
SCF_GUESS gwh
SCF_GUESS_MIX 2
\$end

Here is the oxygen dissociation lcurve:

After that, I adopted the INTERNAL_STABILITY = TRUE in my input to perform internal stability analysis and generate the corrected MOs. But the SCF failed to converge.

Can someone explain why is this happening? Is this keyword( INTERNAL_STABILITY) equivalent to stable=opt in Gaussian?

Thanks a lot!

Looks like there’s a lower-energy solution to which your calculation converges intermittently, which is not unusual for highly stretched bonds. The way that I would generate such a curve is to start from the equilibrium distant (or from R=2 A in your case, since the curve is well-behaved up to that point) and then compute the longer bond lengths by reading in the MOs from the previous (shorter R) calculation and using the maximum overlap method to attempt to find a solution that looks like the one at the shorter R. I’ll paste an example below, but a couple of other things about your input file that are strange are:
(1) why set JOBTYPE=FORCE just to compute a potential energy scan? Do you need the gradient?
(2) SCF_GUESS = GWH is an odd choice, was there a reason?
(3) Your memory settings much higher than needed for such a small molecule

\$comment
First, R=2.00 A
\$end

\$molecule
0 1
O
O 1 2.00
\$end

\$rem
method wB97M-V
basis cc-pVTZ
unrestricted true
scf_guess_mix 5
\$end

@@@

\$comment
Now, R = 2.25 A
\$end

\$molecule
0 1
O
O 1 2.25
\$end

\$rem
method wB97M-V
basis cc-pVTZ
unrestricted true
mom_method imom ! usually works better than original MOM
mom_start 1 ! begin MOM immediately on 1st SCF cycle
\$end

@@@

\$comment
Finally, R = 2.50 A
\$end

\$molecule
0 1
O
O 1 2.50
\$end

\$rem
method wB97M-V
basis cc-pVTZ
unrestricted true
mom_method imom ! usually works better than original MOM
mom_start 1 ! begin MOM immediately on 1st SCF cycle
\$end

Using the approach above, starting from R=2.0 A in 0.25 A increments, I get the following smooth potential energy curve for the singlet state (which is not the ground state for O2…)