Hello everyone!

When calculating the SCF (STO-3G) for small diatomic molecules (hydrogen molecules or helium dimer), I noticed that as the distance between atoms increases, the MO coefficients change in such a way as if at some point there is a switch from RHF to UHF.

For example, MO coefficients for a hydrogen molecule at 21 rBohr are

[[ 0.70710678 0.70710678]

[ 0.70710678 -0.70710678]]

and for 22 rBohr

[[1. 0.]

[0. 1.]].

Does anyone know exactly what setting needs to be changed to make the program work in RHF even over very long distances? Or maybe something else is going on?

There is a full script:

```
import psi4
import numpy as np
psi4.set_memory(int(5e8))
numpy_memory = 2
h2 = psi4.geometry("""
H
H 1 21.0
units=bohr
symmetry c1
""")
psi4.set_options({'basis': 'sto-3g',
'scf_type': 'pk',
'e_convergence': 1e-15
},)
e, wfn = psi4.energy('scf', return_wfn=True)
Matrix_C = wfn.Ca()
C = Matrix_C.np
print(C)
```

Thanks in advance.