AB-INITIO COMPUTATION OF SEMIEMPIRICAL PI-ELECTRON METHODS .4. TRUE AND APPROXIMATE EFFECTIVE-HAMILTONIANS FOR HEXATRIENE AND RELATED CONJUGATED POLYENES

The multireference configuration ab initio effective valence shell Ham iltonian (H-nu) method is used to compute the true pi-electron effecti ve Hamiltonian of hexatriene, to provide new theoretical predictions o f excited states and to further our understanding of semiempirical pi- electron theory. The H-nu approach generates highly accurate vertical excitation energies-for the low-lying valencelike states with a single large-scale calculations using the same valence orbitals for every ex cited state. We present a fairly automated procedure for choosing H-nu valence orbitals and determine a single H-nu six valence orbital vale nce space that reproduces experimental vertical excitation energies to within 0.26, 0.01, 0.08, 0.04, and 0.03 eV for the 1(1)B(u), 1(1)A(g) , 2(1)B(u), 1(3)B(u), and 1(3)A(g) valencelike excited states. The cal culations also provide the first predictions for the 2(3)B(u) and 2(3) A(g) state vertical excitation energies (5.22 and 6.69 eV, respectivel y). These results compare extremely well with complete active space se cond-order perturbation theory calculations (CASPT2) which employ larg er active spaces. A second valence space (constructed for comparisons with semiempirical pi-electron theory) treats most low-lying excited s tates well but also reproduces the 1(2)A(u) state ionization potential to with 0.08 eV. This allows us to predict for the first time the sec ond pi-electron ionization potential to lie at 10.34 eV. Given our hig hly accurate ab initio model, we then apply a series of systematic app roximations which reduce the ab initio pi-electron H-nu to a purely ab initio PPP Hamiltonian (denoted as H-nu-PPP). The new H-nu-PPP out pe rforms the traditional semiempirical PPP parametrization. The ab initi o H-nu-PPP parameters compare well with the standard semiempirical val ues and display remarkable transferability between ethylene, trans-but adiene, and hexatriene. Many ab initio parameters, which are neglected in the semiempirical PPP model, are actually quite small, but only wh en including correlation contributions. On the other hand, contributio ns from many nonnegligible ab initio one-, two-, and three-electron pa rameters must still be retained in order to improve the accuracy of th e ab initio PPP model. We present an improved, generalized PPP model t hat includes all dominant many-body interactions (greater than 0.1 eV) .