Multiplicity in Molecule Keywords

Dear colleagues

I have a question about multiplicity.

I took note that Psi4 determines how many unpaired electrons a molecule has by looking at the user-specified spin multiplicity. Ok.

My question is: what does Psi4 consider if I don’t provide neither the charge nor the multiplicity in the script like as:

molecule{
atom1_molecule1 x y z
atom2_molecule1 x y z
...
--
atom1_molecule2 x y z
atom2_molecule2 x y z
...
}

The “correct” one would be below:

molecule{
charge multiplicity
atom1_molecule1 x y z
atom2_molecule1 x y z
...
--
charge multiplicity
atom1_molecule2 x y z
atom2_molecule2 x y z
...
}

Thanks in advance.
.

It defaults to 0 1. You have a neutral molecule. It is a spin-singlet. Because all single-determinant/single-MO-diagram spin-singlets have all spins paired, Psi will seek SCF solutions with all spins paired.

Many thanks for the response!

Best regards.