How to obtain the One-electron overlap matrix

1

How can I obtain the One-electron overlap matrix

wfn.S().to_array()

from the pairwise overlap of the basis functions?

I noticed that the overlap matrix as defined above is zero for basis functions of the same atom.
Is this correct?

2

wfn.S is your normalized AO overlap matrix that gets added to the core Hamiltonian. Diagonal elements are 1.

Not sure what you mean, but does it answer your question?

3

Do I understand correctly, that wnf.S() can be computed without the SCF converged coefficients?
How can I then obtain the overlap matrix (for different basis sets) without invoking the time comsuming
SCF method?

4

you can built a wavefunction object without doing an scf. This works for me:

mol=psi4.geometry('H -.5 0 0\nH .5 0 0')

set global {
basis def2-SVP
}
scf_wfn=psi4.core.Wavefunction.build(mol, psi4.core.get_global_option('BASIS'))
mints = MintsHelper(scf_wfn.basisset())
ao=mints.ao_overlap()
S = np.array(ao)
np.savetxt('overlap', S)

You want to check here: https://github.com/psi4/psi4numpy/blob/master/Self-Consistent-Field/RHF.py

Since you dont get a fully constructed wfn object back as you would from a scf calculation, I dont think that wfn.S works, and you need to use MintsHelper as above. But…I am also a beginner in this :wink:

5

How can I do the same code in python?

6

Hmm, for that you need to adapt the syntax. Check instructions and examples here https://github.com/psi4/psi4numpy and here http://www.psicode.org/psi4manual/1.1/psiapi.html