The spatial extension of the C2H4 (1)(pi,pi*) V state is investigated by means of low selection threshold multireference configuration interaction (CI) calculations employing two atomic orbital (AO) basis sets with different numbers of polarization and Rydberg functions. The results are shown to be nearly independent of the choice of one-electron basis (ground N, triplet T, and singlet V self-consistent field molecular orbitals (SCF MOs)) in forming the many-electron basis for the configuration interaction indicating that the AO basis limit has been closely approached in each case. The calculations indicate that the value for the [Psi(V)Sigma x(i)(2)Psi(V)]=[x(2)](V) matrix element falls in the 18+/-1 a(0)(2) range, 50% larger than the corresponding values computed for [x(2)](N) and [x(2)](T), respectively, for the corresponding N and T states. This result is interpreted to be a consequence of the mixing of diabatic (1)(pi,pi*) valence and (1)(pi,d pi) Rydberg states in the Franck-Condon region of the V-N transition. The corresponding excitation energy is computed to lie in the 7.90-7.95 eV range, indicating that there is a distinct nonverticality in the measured absorption spectrum which is caused in part by nonadiabatic interactions between the V and (1)(pi,3p(y)) Rydberg states as a result of torsional motion of the C2H4 molecule.