The multireference configuration ab initio effective valence shell Hamiltonian (H-nu) method is used to compute the true pi-electron effective Hamiltonian of hexatriene, to provide new theoretical predictions of 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 excited state. We present a fairly automated procedure for choosing H-nu valence orbitals and determine a single H-nu six valence orbital valence 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 calculations 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, respectively). These results compare extremely well with complete active space second-order perturbation theory calculations (CASPT2) which employ larger active spaces. A second valence space (constructed for comparisons with semiempirical pi-electron theory) treats most low-lying excited states 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 second pi-electron ionization potential to lie at 10.34 eV. Given our highly accurate ab initio model, we then apply a series of systematic approximations which reduce the ab initio pi-electron H-nu to a purely ab initio PPP Hamiltonian (denoted as H-nu-PPP).