Redesigning semiempirical-like pi-electron theory with second order effective valence shell Hamiltonian (Hν) theory: application to large protonated Schiff bases

The fast and simple second order effective valence shell Hamiltonian (Hν) method is applied to the series of protonated Schiff bases (CC−)nCNH2 + for n = 1, 5, generating ab initio analogs of the π-electron effective integrals αi, βi,j and γi,j that appear in the semiempirical Pariser-Pan-Pople (PPP) theory. The Hν βi,j and γi,i transfer remarkably well between all five Schiff bases. The other non-negligible effective integrals constitute only a minute fraction of the matrix elements of the Hν two- and three-electron operators and also transfer exceptionally well. The present calculations set the stage for developing an ab initio PPP model applicable to these and other biological interesting chromophores.