Wavefunction Projection

Sometimes — especially when transition metals or diffuse basis functions are present — the SCF procedure fails to converge because the initial guess is not accurate enough. In such cases it often helps to first converge the wavefunction in a smaller basis set (minimal, sto-3g, etc.) with no diffuse functions, project that solution onto the desired basis set, and then start a fresh SCF using the projected wavefunction as the initial guess.

Use run project together with projectbasis to specify the larger target basis. In the example below the SCF is converged in sto-3g and the result is projected onto 6-31g**:

run             project
basis           sto-3g
projectbasis    6-31g**
charge          0
coordinates     water.xyz
end

The projected wavefunction is written to scr/prjct. Point guess at that file in a subsequent calculation that runs in the larger basis set:

run             energy
guess           scr/prjct
basis           6-31g**
charge          0
coordinates     water.xyz
end

This second job uses the projected wavefunction as its initial guess, which is typically much closer to the large-basis solution than a from-scratch guess.

CASSCF orbital projection

A separate keyword, mcscfprojectbasis, performs the analogous projection of CASSCF/MCSCF orbitals onto a larger basis.